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>
diff --git a/docs/featuresbyrelease/vpp17.04.rst b/docs/featuresbyrelease/vpp17.04.rst
index f62b880..ca26ddc 100644
--- a/docs/featuresbyrelease/vpp17.04.rst
+++ b/docs/featuresbyrelease/vpp17.04.rst
@@ -55,14 +55,14 @@
    -  CGN (Deterministic and dynamic)

    -  CGN configurable port allocation algorithm

    -  ICMP support

-   -  Tentant VRF id for SNAT outside addresses

+   -  Tenant VRF id for SNAT outside addresses

    -  Session dump / User dump

    -  Port allocation per protocol

 * Security groups

    -  Routed interface support

    -  L2+L3 unified processing node

    -  Improve fragment handling

-* Segement routing v6

+* Segment routing v6

    -  SR policies with weighted SID lists

    -  Binding SID

    -  SR steering policies