MOTOR-1358 Fix AgnosticGridFSBucket collection type (#299)

Co-authored-by: Shane Harvey <[email protected]>

Author: jochman

motor/motor_gridfs.pyi

@@ -172,7 +172,7 @@ class AgnosticGridFSBucket:
         chunk_size_bytes: int = ...,
         write_concern: Optional[WriteConcern] = None,
         read_preference: Optional[_ServerMode] = None,
-        collection: Optional[AgnosticCollection] = None,
+        collection: Optional[str] = None,
     ) -> None: ...
     def get_io_loop(self) -> Any: ...
     def wrap(self, obj: Any) -> Any: ...