* doc/contributing.rdoc: [DOC] Fix typo by @dvsuresh [Fixes GH-476]

https://github.com/ruby/ruby/pull/476
[ci-skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44162 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

doc/contributing.rdoc

@@ -207,7 +207,7 @@ Slides should be:
 * Include a corresponding ticket number
 * MUST include a figure and/or short example code
 * SHOULD have less sentence in natural language (try to write less than 140 characters)
-* It is RECOMMENDED to itemize: motivation/use case, propoasl, pros/cons, corner case
+* It is RECOMMENDED to itemize: motivation/use case, proposal, pros/cons, corner case
 * PDF or Image (Web browsers can show it)
 
 Please note: