Typo in contributing.md.

#228

PiperOrigin-RevId: 308841046

Author: taiya

CONTRIBUTING.md

@@ -20,7 +20,7 @@ again.
 All submissions, including submissions by project members, require review. We
 use GitHub pull requests for this purpose. Consult
 [GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
-information on using pull requests.two.
+information on using pull requests.
 
 ## What features to add?