Fix a namespace nit in folly/futures/Future.cpp.

Summary: [Folly] Fix a namespace nit in folly/futures/Future.cpp.

As soon as someone introduces a namespace `folly::futures::detail` this will blow up. Fix it so it won't blow up when that happens.

Reviewed By: @​hannesr

Differential Revision: D2258799

folly/futures/Future.cpp

@@ -33,7 +33,7 @@ namespace folly { namespace futures {
 
 Future<Unit> sleep(Duration dur, Timekeeper* tk) {
   if (LIKELY(!tk)) {
-    tk = detail::getTimekeeperSingleton();
+    tk = folly::detail::getTimekeeperSingleton();
   }
   return tk->after(dur);
 }