#4611: fix typo.

Doc/extending/extending.rst

@@ -865,7 +865,7 @@ memory and should be avoided completely. [#]_
 The advantage of borrowing over owning a reference is that you don't need to
 take care of disposing of the reference on all possible paths through the code
 --- in other words, with a borrowed reference you don't run the risk of leaking
-when a premature exit is taken.  The disadvantage of borrowing over leaking is
+when a premature exit is taken.  The disadvantage of borrowing over owning is
 that there are some subtle situations where in seemingly correct code a borrowed
 reference can be used after the owner from which it was borrowed has in fact
 disposed of it.