Add mention of Token module to help text

Issue Summary

The help text says: "Use the syntax [token] if you want to insert a replacement pattern." Being fairly new to Drupal 7 and not quite knowing how all the new stuff works, I got a bit confused. I literally put "[token]" there thinking that would trigger some other options to show up to put in the replacement pattern. After a bit of muddling around, I realized that Token hadn't been added to core like I thought it had and that installing that makes actual tokens show up underneath which makes it all much clearer. :)

Anyway, my small suggested change is to make the text read: "Use the syntax [token] if you want to insert a replacement pattern using the tokens below (requires Token module)."

Just a little thing but makes it clearer when you don't have Token installed.

Thanks,

Michelle