Fixup the simple plugin setup docs.
Fixup the simple plugin setup docs.

Testing Done:
Rendered here:
  http://pantsbuild.github.io/staging/jsirois/issues/3877/howto_plugin.html

CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/160488897

Bugs closed: 3877, 3878

Reviewed at https://rbcommons.com/s/twitter/r/4241/

peiyuwang/pants,foursquare/pants,pombredanne/pants,cevaris/pants,twitter/pants,mateor/pants,benjyw/pants,ericzundel/pants,UnrememberMe/pants,mateor/pants,benjyw/pants,wisechengyi/pants,jsirois/pants,ericzundel/pants,foursquare/pants,fkorotkov/pants,wisechengyi/pants,peiyuwang/pants,pombredanne/pants,jsirois/pants,mateor/pants,foursquare/pants,lahosken/pants,foursquare/pants,lahosken/pants,wisechengyi/pants,cevaris/pants,pantsbuild/pants,wisechengyi/pants,foursquare/pants,landism/pants,tdyas/pants,pombredanne/pants,kwlzn/pants,15Dkatz/pants,wisechengyi/pants,peiyuwang/pants,UnrememberMe/pants,baroquebobcat/pants,UnrememberMe/pants,landism/pants,mateor/pants,UnrememberMe/pants,cevaris/pants,wisechengyi/pants,wisechengyi/pants,peiyuwang/pants,15Dkatz/pants,UnrememberMe/pants,twitter/pants,landism/pants,wisechengyi/pants,kwlzn/pants,mateor/pants,lahosken/pants,cevaris/pants,twitter/pants,pombredanne/pants,ericzundel/pants,cevaris/pants,tdyas/pants,UnrememberMe/pants,pombredanne/pants,fkorotkov/pants,cevaris/pants,fkorotkov/pants,ericzundel/pants,peiyuwang/pants,baroquebobcat/pants,landism/pants,lahosken/pants,baroquebobcat/pants,baroquebobcat/pants,landism/pants,pantsbuild/pants,baroquebobcat/pants,fkorotkov/pants,tdyas/pants,ericzundel/pants,peiyuwang/pants,kwlzn/pants,kwlzn/pants,pantsbuild/pants,pombredanne/pants,fkorotkov/pants,15Dkatz/pants,tdyas/pants,tdyas/pants,foursquare/pants,landism/pants,pantsbuild/pants,landism/pants,15Dkatz/pants,cevaris/pants,lahosken/pants,mateor/pants,pantsbuild/pants,foursquare/pants,pantsbuild/pants,twitter/pants,fkorotkov/pants,twitter/pants,benjyw/pants,tdyas/pants,tdyas/pants,benjyw/pants,mateor/pants,twitter/pants,UnrememberMe/pants,fkorotkov/pants,lahosken/pants,twitter/pants,landism/pants,lahosken/pants,kwlzn/pants,twitter/pants,foursquare/pants,kwlzn/pants,baroquebobcat/pants,baroquebobcat/pants,UnrememberMe/pants,lahosken/pants,15Dkatz/pants,benjyw/pants,UnrememberMe/pants,15Dkatz/pants,benjyw/pants,tdyas/pants,ericzundel/pants,pombredanne/pants,tdyas/pants,ericzundel/pants,jsirois/pants,15Dkatz/pants,pantsbuild/pants,peiyuwang/pants,twitter/pants,peiyuwang/pants,wisechengyi/pants,15Dkatz/pants,benjyw/pants,foursquare/pants,baroquebobcat/pants,ericzundel/pants,fkorotkov/pants,baroquebobcat/pants,kwlzn/pants,mateor/pants