Fix data race/UB in RCU documentation example (#1348)
Fix data race/UB in RCU documentation example (#1348)

Summary:
The read of globalConfigData outside of sm.lock() races with any other writer that is modifying
globalConfigData inside of sm.lock(). This is a documentation only change.
Pull Request resolved: https://github.com/facebook/folly/pull/1348

Reviewed By: paulmckrcu

Differential Revision: D20864073

Pulled By: yfeldblum

fbshipit-source-id: 31b7ed469716acb35913df9a71585c2a59e50bea

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly