fix Baton.h typo

Summary: checked the sense
Closes #224

Reviewed By: @paulbiss

Differential Revision: D2144508

Pulled By: @sgolemon

folly/Baton.h

@@ -51,7 +51,7 @@ struct Baton : boost::noncopyable {
   /// wait()ing.  In practice this means that the waiter usually takes
   /// responsibility for destroying the Baton.
   ~Baton() {
-    // The docblock for this function says that is can't be called when
+    // The docblock for this function says that it can't be called when
     // there is a concurrent waiter.  We assume a strong version of this
     // requirement in which the caller must _know_ that this is true, they
     // are not allowed to be merely lucky.  If two threads are involved,