Rename tag modular-form to modular-forms

I hope everyone agrees that the plural is a better tag.

• I've gone ahead and retagged; probably a mod can remove the other one. – J. M. isn't a mathematician Nov 22 '10 at 2:14
• I think that moderators have the ability to merge tags, so that bumping to the front page is unnecessary. – Jonas Meyer Nov 22 '10 at 2:22
• Well, there were only five questions using that tag; so not much of a problem IMHO. On the other hand, if it were as big a tag as number-theory ... – J. M. isn't a mathematician Nov 22 '10 at 3:07