Fix typo in Hazptr (#1611)

Summary: Pull Request resolved: https://github.com/facebook/folly/pull/1611

Reviewed By: magedm

Differential Revision: D29938492

Pulled By: yfeldblum

fbshipit-source-id: d0fab63bdb59e714baaee2be15139f04958672d5

folly/synchronization/Hazptr.h

@@ -59,7 +59,7 @@
 ///   protected by hazard pointers.
 /// - The essential components of the hazptr API are:
 ///   o hazptr_holder: Class that owns and manages a hazard pointer.
-///   o protect: Mmember function of hazptr_holder. Protects
+///   o protect: Member function of hazptr_holder. Protects
 ///     an object pointed to by an atomic source (if not null).
 ///       T* protect(const atomic<T*>& src);
 ///   o hazptr_obj_base<T>: Base class for protected objects.