Update branches names in CONTRIBUTING:md (#3943)

Author: bryant1410

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 # Contributing to RxJava
 
-If you would like to contribute code you can do so through GitHub by forking the repository and sending a pull request (on a branch other than `master`, `0.x`, `1.x`, or `gh-pages`).
+If you would like to contribute code you can do so through GitHub by forking the repository and sending a pull request (on a branch other than `master`, `1.x`, `2.x`, or `gh-pages`).
 
 When submitting code, please make every effort to follow existing conventions and style in order to keep the code as readable as possible.