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

folly/Random.h

@@ -40,7 +40,7 @@ namespace folly {
  * However, if you are worried about performance, you can memoize the TLS
  * lookups that get the per thread state by manually using this class:
  *
- * ThreadLocalPRNG rng = Random::threadLocalPRNG()
+ * ThreadLocalPRNG rng;
  * for (...) {
  *   Random::rand32(rng);
  * }