Update packages/google-cloud-firestore/google/cloud/firestore_v1/collection.py

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: daniel-sanche

packages/google-cloud-firestore/google/cloud/firestore_v1/collection.py

@@ -136,7 +136,7 @@ def add(
         return write_result.update_time, document_ref
 
 
-    def document(self, document_id: str | None = None) -> DocumentReference:
+    def document(self, document_id: Union[str, None] = None) -> "DocumentReference":
         """Create a sub-document underneath the current collection.
 
         Args: