Add None to set_prop_cycle docstring

Add `None` to the docstring as a request from the maintainers.

Author: vnmabus

Diff for: lib/matplotlib/axes/_base.py

@@ -1600,7 +1600,7 @@ def set_prop_cycle(self, *args, **kwargs):
 
         Parameters
         ----------
-        cycler : `~cycler.Cycler`
+        cycler : `~cycler.Cycler` or ``None``
             Set the given Cycler. *None* resets to the cycle defined by the
             current style.