Fixup invoking.md to refer to `--config-override`.
Fixup invoking.md to refer to `--config-override`.

Previously it referred to a pluralized verison of the flag.

Testing Done:
Published here:
  http://pantsbuild.github.io/staging/jsirois/invoking.html#overlay-ini-files-with-config-override

CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/90989394

Bugs closed: 2555, 2556

Reviewed at https://rbcommons.com/s/twitter/r/3115/

lahosken/pants,lahosken/pants,wisechengyi/pants,ity/pants,wisechengyi/pants,ericzundel/pants,baroquebobcat/pants,dturner-tw/pants,wisechengyi/pants,benjyw/pants,ity/pants,15Dkatz/pants,ity/pants,twitter/pants,baroquebobcat/pants,gmalmquist/pants,cevaris/pants,mateor/pants,dbentley/pants,lahosken/pants,cevaris/pants,twitter/pants,foursquare/pants,tdyas/pants,foursquare/pants,pantsbuild/pants,jtrobec/pants,benjyw/pants,fkorotkov/pants,tdyas/pants,wisechengyi/pants,pantsbuild/pants,tdyas/pants,baroquebobcat/pants,benjyw/pants,wisechengyi/pants,ericzundel/pants,pantsbuild/pants,peiyuwang/pants,15Dkatz/pants,kwlzn/pants,pombredanne/pants,cevaris/pants,fkorotkov/pants,baroquebobcat/pants,mateor/pants,benjyw/pants,gmalmquist/pants,ericzundel/pants,manasapte/pants,pombredanne/pants,wisechengyi/pants,ity/pants,landism/pants,jtrobec/pants,twitter/pants,landism/pants,pombredanne/pants,peiyuwang/pants,baroquebobcat/pants,foursquare/pants,jsirois/pants,mateor/pants,peiyuwang/pants,mateor/pants,ericzundel/pants,manasapte/pants,15Dkatz/pants,lahosken/pants,manasapte/pants,jtrobec/pants,gmalmquist/pants,gmalmquist/pants,kwlzn/pants,tdyas/pants,lahosken/pants,foursquare/pants,15Dkatz/pants,baroquebobcat/pants,benjyw/pants,wisechengyi/pants,mateor/pants,dturner-tw/pants,manasapte/pants,tdyas/pants,tdyas/pants,dturner-tw/pants,baroquebobcat/pants,twitter/pants,twitter/pants,foursquare/pants,15Dkatz/pants,15Dkatz/pants,UnrememberMe/pants,pantsbuild/pants,jsirois/pants,pombredanne/pants,landism/pants,foursquare/pants,kwlzn/pants,peiyuwang/pants,foursquare/pants,dturner-tw/pants,ericzundel/pants,pombredanne/pants,15Dkatz/pants,wisechengyi/pants,cevaris/pants,cevaris/pants,peiyuwang/pants,tdyas/pants,ity/pants,dbentley/pants,ericzundel/pants,kwlzn/pants,ity/pants,gmalmquist/pants,tdyas/pants,pombredanne/pants,fkorotkov/pants,lahosken/pants,peiyuwang/pants,pantsbuild/pants,landism/pants,tdyas/pants,UnrememberMe/pants,foursquare/pants,peiyuwang/pants,dbentley/pants,dbentley/pants,dturner-tw/pants,twitter/pants,pantsbuild/pants,UnrememberMe/pants,fkorotkov/pants,UnrememberMe/pants,lahosken/pants,landism/pants,benjyw/pants,manasapte/pants,landism/pants,fkorotkov/pants,jsirois/pants,dturner-tw/pants,ity/pants,UnrememberMe/pants,fkorotkov/pants,cevaris/pants,mateor/pants,kwlzn/pants,landism/pants,gmalmquist/pants,UnrememberMe/pants,benjyw/pants,mateor/pants,mateor/pants,gmalmquist/pants,jtrobec/pants,lahosken/pants,baroquebobcat/pants,jtrobec/pants,fkorotkov/pants,dbentley/pants,jtrobec/pants,15Dkatz/pants,manasapte/pants,dturner-tw/pants,foursquare/pants,jtrobec/pants,kwlzn/pants,twitter/pants,landism/pants,wisechengyi/pants,ericzundel/pants,manasapte/pants,UnrememberMe/pants,peiyuwang/pants,cevaris/pants,kwlzn/pants,pombredanne/pants,dbentley/pants,twitter/pants,fkorotkov/pants,UnrememberMe/pants,UnrememberMe/pants,pantsbuild/pants,dbentley/pants,twitter/pants,ericzundel/pants,baroquebobcat/pants