CONTRIBUTING: English fixes for PR rebase section

Add a few fixes from a native English speaker to clarify the "how do changes suggested on a pull request" section. In particular, re-word the title which was non-idiomatic.

Other edits are minor: clarify "it" in one place and add a missing definite article.

Author: rjw57

CONTRIBUTING.md

@@ -43,9 +43,9 @@ $ git push origin improve-contacts-99
 
 * Do a new pull request from your "improve-contacts-99" branch to webogram "master".
 
-#### How do changes suggested on a pull request
+#### How to implement changes suggested on a pull request
 
-Some times when you do a PR, you will be asked to correct some code. You can do it on your work branch and commit normally, PR will be automatically updated.
+Sometimes when you submit a PR, you will be asked to correct some code. You can make the changes on your work branch and commit normally and the PR will be automatically updated.
 
 ``$ git commit -am "Ops, fixing typo"``