docs: add spellcheck to 'make docs' sphinx docs

The CI gate will fail if there are typos in the docs.

  writing output... [ 21%] events/Summits/OpensourceSummit...
  writing output... [ 22%] events/Summits/UKNO/2017_04_30_...

  featuresbyrelease/vpp16.06.rst:34:Rasberry:vpp16.06
  writing output... [100%] usecases/vppinazure
  Spelling checker messages written to /vpp/docs/_build/html/output.txt

  Warning, treated as error:
  Found 1 misspelled words
  Makefile:31: recipe for target 'html' failed
  make[1]: *** [html] Error 2
  make[1]: Leaving directory '/vpp/docs'

If you introduce a term that is not recognized,
please add it to custom dictionary at
docs/spelling_wordlist.txt.

Type: feature

Change-Id: Id49be4fbee617f544f1ab8e78e7de8a4df36448b
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>

3 files changed, 3 insertions, 3 deletions

diff --git a/docs/usecases/ConnectingVPC.rst b/docs/usecases/ConnectingVPC.rst
index cc8c927b2cb..f5b2c5dc9ae 100644
--- a/docs/usecases/ConnectingVPC.rst
+++ b/docs/usecases/ConnectingVPC.rst
@@ -25,7 +25,7 @@ In our VPC we will have two instances: one, in which we will install VPP and the
 Figure 2: Example of the resources present inside our VPC
 
 
-Notice that the following example works with two VPCs, where in each of them there are a VM with VPP and a VM. Hence,  you will have to execute the same commands also in the other VPC to make the connection between the two VPC possibile.
+Notice that the following example works with two VPCs, where in each of them there are a VM with VPP and a VM. Hence,  you will have to execute the same commands also in the other VPC to make the connection between the two VPC possible.
 
 
 Now, create a new VM instance (you can use same setting as before (Ubuntu Server 16.04 and m5 type)) and attach a NIC. Remember that the two Client/Server machine's NICs should stay in two different IPv4 Subnet. Afterwards, on the VM's terminal execute these commands:
diff --git a/docs/usecases/contiv/K8s_Overview.md b/docs/usecases/contiv/K8s_Overview.md
index ba3a8e12524..69f144b2953 100644
--- a/docs/usecases/contiv/K8s_Overview.md
+++ b/docs/usecases/contiv/K8s_Overview.md
@@ -6,7 +6,7 @@
 Kubernetes is a container orchestration system that efficiently manages Docker containers. The Docker containers and container platforms provide many advantages over traditional virtualization. Container isolation is done on the kernel level, which eliminates the need for a guest virtual operating system, and therefore makes containers much more efficient, faster, and lightweight. The containers in Contiv/VPP are referred to as PODs.
 
 Contiv/VPP is a Kubernetes network plugin that uses [FD.io VPP](https://fd.io/) 
-to provide network connectivity between PODs in a k8s cluster (k8s is an abbreviated reference for kubernates).
+to provide network connectivity between PODs in a k8s cluster (k8s is an abbreviated reference for kubernetes).
 It deploys itself as a set of system PODs in the `kube-system` namespace,
 some of them (`contiv-ksr`, `contiv-etcd`) on the master node, and some
 of them (`contiv-cni`, `contiv-vswitch`, `contiv-stn`) on each node in the cluster. 
diff --git a/docs/usecases/simpleperf/iperf3.rst b/docs/usecases/simpleperf/iperf3.rst
index 0b0de373bff..6f5d345c598 100644
--- a/docs/usecases/simpleperf/iperf3.rst
+++ b/docs/usecases/simpleperf/iperf3.rst
@@ -3,7 +3,7 @@
 Introduction
 ============
 
-This tutorial shows how to use VPP use iperf3 and Trex to get some basic performance
+This tutorial shows how to use VPP use iperf3 and TRex to get some basic performance
 numbers from a few basic configurations. Four examples are shown. In the first two
 examples, the **iperf3** tool is used to generate traffic, and in the last two examples
 the Cisco's `TRex Realistic Traffic Generator <http://trex-tgn.cisco.com/>`_ is used. For