Fix openapi schema method mapping

Fix incorrect schema method mapping for "patch" whose action
is called "partial_update".

rest_framework/schemas/openapi.py

@@ -137,7 +137,7 @@ class AutoSchema(ViewInspector):
         'get': 'retrieve',
         'post': 'create',
         'put': 'update',
-        'patch': 'partialUpdate',
+        'patch': 'partial_update',
         'delete': 'destroy',
     }