Fix annotation for group_instance_id parameter (#2606)

Author: gandhis1

faststream/confluent/broker/registrator.py

@@ -115,7 +115,7 @@ def subscriber(
         partitions: Sequence["TopicPartition"] = (),
         polling_interval: float = 0.1,
         group_id: str | None = None,
-        group_instance_id: None = None,
+        group_instance_id: str | None = None,
         fetch_max_wait_ms: int = 500,
         fetch_max_bytes: int = 50 * 1024 * 1024,
         fetch_min_bytes: int = 1,