Correct docstring of CheckButtons.get_status

The return type is a list, not a tuple, adjust the docstring to match
implementation.

Author: ksunden

lib/matplotlib/widgets.py

@@ -1084,7 +1084,7 @@ def set_active(self, index):
 
     def get_status(self):
         """
-        Return a tuple of the status (True/False) of all of the check buttons.
+        Return a list of the status (True/False) of all of the check buttons.
         """
         return [l1.get_visible() for (l1, l2) in self.lines]