contributing copy fixup

Author: samccone

CONTRIBUTING.md

@@ -54,7 +54,7 @@ PR's are even better than issues. We gladly accept community pull requests. In g
 - Sign the [CLA](https://cla.developers.google.com/clas), as described above.
 - Fork the repo you're making the fix on to your own Github account.
 - Code!
-- Ideally, squash your commits into a single commit with a clear message of what the PR does. If it absolutely makes sense to keep multiple commits, that OK - or perhaps consider making two separate PR's.
+- Ideally, squash your commits into a single commit with a clear message of what the PR does. If it absolutely makes sense to keep multiple commits, that's OK - or perhaps consider making two separate PR's.
 - **Include tests that test the range of behavior that changes with your PR.** If you PR fixes a bug, make sure your tests capture that bug. If your PR adds new behavior, make sure that behavior is fully tested. Every PR *must* include associated tests.
 - Submit your PR, making sure it references the issue you created.
 - If your PR fixes a bug, make sure the issue includes clear steps to reproduce the bug so we can test your fix.