This should be `validates`, not `validate`

Found this while testing discourse with Rails master. (It's that time of the year again :grin:)

Thanks to https://github.com/rails/rails/pull/16210, this now raises an exception on Rails master! <3 <3 <3

Obviously this was not doing what it's supposed to do before, so there's probably a hole in the test coverage somewhere, but I figure I'll send it first in case someone can do that before I get to it.