Fixed documentation in folly/Random.h
Fixed documentation in folly/Random.h

Summary:
The docs give `ThreadLocalPRNG rng = Random::threadLocalPRNG()` as an example
of creating a thread local PRNG, but `Random::threadLocalPRNG()` does not/no longer
exist. I think it's just `folly::ThreadLocalPRNG` right now.

Reviewed By: yfeldblum

Differential Revision: D5184992

fbshipit-source-id: 63a9ef62b32fca42088abec419ae53531910cc82

Orvid/folly,rklabs/folly,rklabs/folly,Orvid/folly,rklabs/folly,facebook/folly,rklabs/folly,Orvid/folly,facebook/folly,rklabs/folly,facebook/folly,Orvid/folly,Orvid/folly,facebook/folly,facebook/folly