* 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