Only do "_" to " " conversion in labels

I think the change in 22ec5ab3828e1c6a8d356f3a7b244970ab723cd2 is smart for converting form field names into more readable label.  But it was touching the contents of `help` too, which I don't think should have this conversion applied.  This does the same thing, but within the `label()` function, so only Labels are affected.

See comments on [22ec5ab](https://github.com/Anahkiasen/former/commit/22ec5ab3828e1c6a8d356f3a7b244970ab723cd2#commitcomment-4543228) for some history.