Fix typo

Author: zoffixznet

doc/Type/Failure.pod6

@@ -75,7 +75,7 @@ Returns the L<Exception> object that the failure wraps.
 
 Defined as:
 
-    method exception(Failure:D: --> Failure:D)
+    method self(Failure:D: --> Failure:D)
 
 If the invocant is a L<handled> C<Failure>, returns it as is.
 If not handled, throws its L<Exception>. Since