Fix method name in Visitor docstring (#92)

Author: Cito

src/graphql/language/visitor.py

@@ -124,7 +124,7 @@ def enter(self, node, key, parent, path, ancestors):
             # any other value: replace this node with the returned value
             return
 
-        def enter(self, node, key, parent, path, ancestors):
+        def leave(self, node, key, parent, path, ancestors):
             # The return value has the following meaning:
             # IDLE (None) or SKIP: no action
             # BREAK: stop visiting altogether