Fix: Invalid RST bpy.utils.register_cli_command formatting

Titles can't be used because this text is indented.

Ref: !119320

doc/python_api/examples/bpy.utils.register_cli_command.py

@@ -1,6 +1,5 @@
 """
-Custom Commands
----------------
+**Custom Commands**
 
 Registering commands makes it possible to conveniently expose command line
 functionality via commands passed to (``-c`` / ``--command``).