Remove colons in merge message example in contributing doc.

Merging #6 did not close #5, because if there is a colon between 'fixes' and the issue number, github fails to interpret this as a directive to close the issue.

So I modified the contributing doc so the example doesn't have a colon either, and point this lack of colon out as well.  Since the colon was removed from 'Fixes', I also removed the colon after "Reviewers" because that looks strange.