network get_rec_step_info, small doc fix

Author: albertz

returnn/tf/network.py

@@ -2197,7 +2197,7 @@ def have_rec_step_info(self):
   def get_rec_step_info(self, must_exist=True):
     """
     :param bool must_exist: if True, will throw exception if not available
-    :rtype: TFNetworkRecLayer.RecStepInfoLayer|None
+    :rtype: returnn.tf.layers.rec.RecStepInfoLayer|None
     """
     # noinspection PyProtectedMember
     from returnn.tf.layers.rec import RecStepInfoLayer, _SubnetworkRecCell