fix typo and use "GitHub" instead of "github" at CONTRIBUTING.md

git-svn-id: https://svn.redmine.org/redmine/trunk@13621 e93f8b46-1217-0410-a6f0-8f06a7374b81

CONTRIBUTING.md

 
-**Do not send a pull requst to this github repository**.
+**Do not send a pull request to this GitHub repository**.
 
 For more detail, please see [official website] wiki [Contribute].