Administration: Add missing comma to the message in "Post via email" section.

Props kinjaldalwadi, sabernhardt.
Fixes #49238.
Built from https://develop.svn.wordpress.org/trunk@48186


git-svn-id: https://core.svn.wordpress.org/trunk@47955 1a063a9b-81f0-0310-95a4-ce76da25c4cd