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/vpp17.07.rst b/docs/featuresbyrelease/vpp17.07.rst
index 9774421..57b3a52 100644
--- a/docs/featuresbyrelease/vpp17.07.rst
+++ b/docs/featuresbyrelease/vpp17.07.rst
@@ -32,7 +32,7 @@
 

 * memif: IP mode, jumbo frames, multi queue

 * virtio-user support

-* vhost-usr; adaptive (poll/interupt) support.

+* vhost-usr; adaptive (poll/interrupt) support.

 

 Network features

 ++++++++++++++++++

@@ -41,13 +41,13 @@
 * BFD FIB integration

 * NAT64 support

 * GRE over IPv6

-* Segement routing MPLS

+* Segment routing MPLS

 * IOAM configuration for SRv6 localsid

 * LISP

    -  NSH support

    -  native forward static routes

    -  L2 ARP

-* ACL multi-core suuport

+* ACL multi-core support

 * Flowprobe:

    -  Add flowstartns, flowendns and tcpcontrolbits

    -  Stateful flows and IPv6, L4 recording