community[patch]: fix RecursiveUrlLoader metadata_extractor return type (#18193)

**Description:** Fix `metadata_extractor` type for `RecursiveUrlLoader`,
the default `_metadata_extractor` returns `dict` instead of `str`.
**Issue:** N/A
**Dependencies:** N/A
**Twitter handle:** N/A

Signed-off-by: Hemslo Wang <hemslo.wang@gmail.com>

libs/community/langchain_community/document_loaders/recursive_url_loader.py

@@ -86,7 +86,7 @@ class RecursiveUrlLoader(BaseLoader):
         max_depth: Optional[int] = 2,
         use_async: Optional[bool] = None,
         extractor: Optional[Callable[[str], str]] = None,
-        metadata_extractor: Optional[Callable[[str, str], str]] = None,
+        metadata_extractor: Optional[Callable[[str, str], dict]] = None,
         exclude_dirs: Optional[Sequence[str]] = (),
         timeout: Optional[int] = 10,
         prevent_outside: bool = True,