fix atomic::store bad usage

Summary:
Fix usage of atomic::store() where memory order is the only argument.

It worked because of memory_order value is implicit casted to correct value luckily.

Reviewed By: djwatson

Differential Revision: D8108348

fbshipit-source-id: 7c4050b8cb50ec3ab6977cd9adce45249130320c

folly/executors/ThreadPoolExecutor.cpp

@@ -234,7 +234,7 @@ void ThreadPoolExecutor::join() {
     n = threadList_.get().size();
     removeThreads(n, true);
     n += threadsToJoin_.load(std::memory_order_relaxed);
-    threadsToJoin_.store(std::memory_order_relaxed);
+    threadsToJoin_.store(0, std::memory_order_relaxed);
   }
   joinStoppedThreads(n);
   CHECK_EQ(0, threadList_.get().size());