Update session_id docstring (#1304)

Author: kt474

qiskit_ibm_runtime/session.py

@@ -275,7 +275,7 @@ def session_id(self) -> str:
         """Return the session ID.
 
         Returns:
-            Session ID. None until a job runs in the session.
+            Session ID. None until a job is submitted.
         """
         return self._session_id