docstring fix

mbtrack2/tracking/beam_ion_effects.py

@@ -353,6 +353,9 @@ class BeamIonElement(Element):
         The number of records in the built-in ion beam monitor. Should be number of turns times number of bunches because the monitor records every turn after each bunch passage.
     n_ion_macroparticles_per_bunch : int, optional
         The number of ion macroparticles generated per electron bunch passed. Defaults to 30.
+    ion_beam_monitor_name : str, optional
+         If provided, the name of the ion monitor output file. It must end with an extension '.hdf5'.
+         If None, no ion monitor file is generated.
     use_ion_phase_space_monitor : bool, optional
         Whether to use the ion phase space monitor.
     generate_method : str, optional