Small typo: environments not envirements in platform.rst (tox-dev#729)

* Small typo: environments not envirements * Adding myself to CONTRIBUTORS per pull request checklist

Author: jaketothepast

CONTRIBUTORS

@@ -24,6 +24,7 @@ Ian Stapleton Cordasco
 Igor Duarte Cardoso
 Ionel Maries Cristian
 Itxaka Serrano
+Jake Windle
 Jannis Leidel
 Johannes Christ
 Josh Smeaton

doc/example/platform.rst

@@ -26,7 +26,7 @@ and the following ``tox.ini`` content:
  mymacos: darwin
  mywindows: win32
 
- # you can specify dependencies and their versions based on platform filtered envirements
+ # you can specify dependencies and their versions based on platform filtered environments
  deps = mylinux,mymacos: py==1.4.32
  mywindows: py==1.4.30