Update guidance on commit messages

Mentions not terminating lines with punctuation

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>

CONTRIBUTING.md

@@ -62,11 +62,12 @@ Please read this whole guide and make sure you agree to our DCO agreement (inclu
 The first line of the commit message is the *subject*, this should be followed by a blank line and then a message describing the intent and purpose of the commit. These guidelines are based upon a [post by Chris Beams](https://chris.beams.io/posts/git-commit/).
 
 * When you run `git commit` make sure you sign-off the commit by typing `git commit -s`.
-* The commit subject should start with an uppercase letter
-* The commit subject should not exceed 72 characters in length
+* The commit subject-line should start with an uppercase letter
+* The commit subject-line should not exceed 72 characters in length
+* The commit subject-line should not end with punctuation (., etc)
 
 When giving a commit body:
-* Leave a blank line after the subject
+* Leave a blank line after the subject-line
 * Make sure all lines are wrapped to 72 characters
 
 Here's an example: