Fix heading of README.

  [Fix GH-1586] Patch by @jbampton

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58393 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Author: hsbt

README.md

@@ -157,7 +157,7 @@ Bug reports should be filed at https://bugs.ruby-lang.org. Read [HowToReport] fo
 
 [HowToReport]: https://bugs.ruby-lang.org/projects/ruby/wiki/HowToReport
 
-##Contributing
+## Contributing
 
 See the file [CONTRIBUTING.md](CONTRIBUTING.md)