Grammar nit. SF bug #757822

Doc/lib/libweakref.tex

@@ -19,7 +19,7 @@ object which is referred to by a weak reference.
 
 XXX --- need to say more here!
 
-Not all objects can be weakly referenced; those objects which do
+Not all objects can be weakly referenced; those objects which can
 include class instances, functions written in Python (but not in C),
 and methods (both bound and unbound).  Extension types can easily
 be made to support weak references; see section \ref{weakref-extension},