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/vpp18.07.rst b/docs/featuresbyrelease/vpp18.07.rst
index b4ee1ae..1f82f5c 100644
--- a/docs/featuresbyrelease/vpp18.07.rst
+++ b/docs/featuresbyrelease/vpp18.07.rst
@@ -13,7 +13,7 @@
 

    - Complete rework of the dpdk-input node

    - Display rx/tx burst function name in "show hardware detail"

-   - Improve buffer alloc perfomance

+   - Improve buffer alloc performance

       + This is ~50% improvement in buffer alloc performance. For a 256 buffer allocation, it was ~10 clocks/buffer, now is < 5 clocks.

    - Add per-numa page allocation info to 'show memory'

    - Vectorized bihash_{48,40,24,16}_8 key compare