docs: fix dead pydantic v1 link in tutorial/response-model.md

`https://docs.pydantic.dev/1.10/usage/exporting_models/#modeldict` (404) —
the pydantic v1 docs site (docs.pydantic.dev/1.10/) was retired after the
v2 docs migration.

Replaced with `https://docs.pydantic.dev/latest/concepts/serialization/#model-dict`
(200) — the current canonical home for the equivalent concept in pydantic v2.
Also updated the anchor from `#modeldict` to `#model-dict` (v2 docs use
hyphenated heading slugs).

docs/en/docs/tutorial/response-model.md

@@ -258,7 +258,7 @@ You can also use:
 * `response_model_exclude_defaults=True`
 * `response_model_exclude_none=True`
 
-as described in [the Pydantic docs](https://docs.pydantic.dev/1.10/usage/exporting_models/#modeldict) for `exclude_defaults` and `exclude_none`.
+as described in [the Pydantic docs](https://docs.pydantic.dev/latest/concepts/serialization/#model-dict) for `exclude_defaults` and `exclude_none`.
 
 ///