Fix *Throttle class names in the docs

* AnonThrottle -> AnonRateThrottle
* UserThrottle -> UserRateThrottle
* ScopedThrottle -> ScopedRateThrottle
* (cosmetic) removed one errant whitespace character

Fixes #1006.
Merge pull request #1007 from willkg/throttle-docs-fix-1006

Fix *Throttle class names in the docs