docs: drop code references to local plan files

config/packages/serializer.yaml

@@ -1,7 +1,7 @@
 # Enabled for #[MapRequestPayload] on the api controllers - nothing else in this application
 # serializes. ALLOW_EXTRA_ATTRIBUTES stays at its default (true) on purpose: the TYPO3 form
 # payload grows keys without a release here, and rejecting an unknown one would turn every
-# later mapping change into a 400 (see .ddev/plans/application-form-endpoint.md).
+# later mapping change into a 400.
 framework:
     serializer:
         name_converter: ~