Fix typo in mutex.h (#917)

src/mutex.h

@@ -71,7 +71,7 @@ typedef std::condition_variable Condition;
 // NOTE: Wrappers for std::mutex and std::unique_lock are provided so that
 // we can annotate them with thread safety attributes and use the
 // -Wthread-safety warning with clang. The standard library types cannot be
-// used directly because they do not provided the required annotations.
+// used directly because they do not provide the required annotations.
 class CAPABILITY("mutex") Mutex {
  public:
   Mutex() {}