Fix BunchMonitor docstring

tracking/monitors/monitors.py

@@ -232,7 +232,7 @@ class Monitor(Element, metaclass=ABCMeta):
 class BunchMonitor(Monitor):
     """
     Monitor a single bunch and save attributes 
-    (mean, std, emit, current, and action).
+    (mean, std, emit, current, and cs_invariant).
     
     Parameters
     ----------