Fix a few typos

CONTRIBUTING

@@ -17,11 +17,11 @@ On authors, maintainers, and contributing:
    don't have time to fix major problems, then you probably shouldn't be
    contributing to the project (or any project) in the first place.
 
- - Be prepared to work with other contibutors.  If you are venturing in to
+ - Be prepared to work with other contributors.  If you are venturing in to
    someone else's code (to improve/expand upon it, or to change it for the
    better), be prepared to talk and work with that person, whomever it may be.
    Do *not* go rewriting someone elses code from scratch just because you feel
-   you can do it better,  Instead, examine what they did, talk with the person
+   you can do it better.  Instead, examine what they did, talk with the person
    (constructively) and if possible work with them to try to improve their
    existing code.