Add icon to the help button in distraction-free writing.

props ocean90.
fixes #26450.


git-svn-id: https://develop.svn.wordpress.org/trunk@26722 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/css/editor.css

@@ -743,6 +743,7 @@ span.mce_redo:before {
 	content: '\f172';
 }
 
+span.mce_help:before,
 span.mce_wp_help:before {
 	content: '\f223';
 }