alembic: Fix typo in add_auto_info_to_endpoint_dtmf_mode

The downgrade function was missing "_v2" at the end of the
alter column type.

Change-Id: Iaa9bcef48d6f3590ce07a61342d8e66f00263d8e

contrib/ast-db-manage/config/versions/164abbd708c_add_auto_info_to_endpoint_dtmf_mode.py

@@ -52,7 +52,7 @@ def downgrade():
         enum.create(op.get_bind(), checkfirst=False)
 
         op.execute('ALTER TABLE ps_endpoints ALTER COLUMN dtmf_mode TYPE'
-                   ' pjsip_dtmf_mode_values USING'
+                   ' pjsip_dtmf_mode_values_v2 USING'
                    ' dtmf_mode::text::pjsip_dtmf_mode_values_v2')
 
         ENUM(name="pjsip_dtmf_mode_values_v3").drop(op.get_bind(), checkfirst=False)