Corrected typo in Doxygen doc (again).

git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@8323 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

FL/Fl.H

@@ -984,7 +984,7 @@ public:
     The thread_message() method returns the last message
     that was sent from a child by the awake() method.
     
-    See also: advanced_multithreading
+    See also: \ref advanced_multithreading
   */
   static void* thread_message(); // platform dependent
   /** @} */