Fix API change message from matplotlib#18989

- Use correct issue number in filename
- Fix typo `non` -> `None`

Author: timhoffm

doc/api/next_api_changes/behavior/18988-TH.rst

@@ -0,0 +1,5 @@
+The Artist property *rasterized* cannot be *None* anymore
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It is now a bool only. Before the default was *None* and
+`.Artist.set_rasterized` documented to accept *None*. However, *None* did not
+have a special meaning and was treated as *False*.