Editing the docstring, making it more consistent to the other parameters that default to None.

Co-authored-by: Ruth Comer <[email protected]>

Author: pedrompecanha

lib/matplotlib/axes/_base.py

@@ -3242,7 +3242,7 @@ def ticklabel_format(self, *, axis='both', style=None, scilimits=None,
         axis : {'x', 'y', 'both'}, default: 'both'
             The axis to configure.  Only major ticks are affected.
 
-        style : {'sci', 'scientific', 'plain'} or None, default: None
+        style : {'sci', 'scientific', 'plain'}
             Whether to use scientific notation.
             The formatter default is to use scientific notation.
             'sci' is equivalent to 'scientific'.