Replace comma with semicolon in CONTRIBUTING.md

Author: eyqs

CONTRIBUTING.md

@@ -6,7 +6,7 @@ just a few small guidelines you need to follow.
 ## Contributor License Agreement
 
 Contributions to this project must be accompanied by a Contributor License
-Agreement. You (or your employer) retain the copyright to your contribution,
+Agreement. You (or your employer) retain the copyright to your contribution;
 this simply gives us permission to use and redistribute your contributions as
 part of the project. Head over to <https://cla.developers.google.com/> to see
 your current agreements on file or to sign a new one.