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