notes/styleguide.rst: Add comma after “e.g.”

To any style-obsessed writers or editors reading this commit:

The comma (or lack of one) after “e.g.” or “i.e.” has always bugged me.  I finally looked it up!  Here’s what I found:

http://english.stackexchange.com/questions/16172/should-i-always-use-a-comma-after-e-g-or-i-e

docs/notes/styleguide.rst

@@ -30,7 +30,7 @@ Strive to keep any contributions relevant to the :ref:`purpose of The Guide
 * `Cite <http://sphinx.pocoo.org/rest.html?highlight=citations#citations>`_
   references where needed.
 * If a subject isn't directly relevant to Python, but useful in conjunction
-  with Python (e.g. Git, GitHub, Databases), reference by linking to useful
+  with Python (e.g., Git, GitHub, Databases), reference by linking to useful
   resources, and describe why it's useful to Python.
 * When in doubt, ask.