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

folly/synchronization/Rcu.h

@@ -67,9 +67,10 @@
 // void writer() {
 //   while (true) {
 //     std::this_thread::sleep_for(std::chrono::seconds(60));
-//     ConfigData* oldConfigData = globalConfigData;
+//     ConfigData* oldConfigData;
 //     ConfigData* newConfigData = loadConfigDataFromRemoteServer();
 //     sm.lock();
+//     oldConfigData = globalConfigData;
 //     globalConfigData = newConfigData;
 //     sm.unlock();
 //     delete oldConfigData;