Fix typo in weakref.proxy() documentation.

This closes SF bug #423087.

Doc/lib/libweakref.tex

@@ -62,7 +62,7 @@ be made to support weak references; see section \ref{weakref-extension},
   callable.  Proxy objects are not hashable regardless of the
   referent; this avoids a number of problems related to their
   fundamentally mutable nature, and prevent their use as dictionary
-  keys.  \var{callable} is the same as the parameter of the same name
+  keys.  \var{callback} is the same as the parameter of the same name
   to the \function{ref()} function.
 \end{funcdesc}