Update extensions primer in line with async breaking changes

docs/ext/commands/extensions.rst

@@ -10,7 +10,7 @@ There comes a time in the bot development when you want to extend the bot functi
 Primer
 --------
 
-An extension at its core is a python file with an entry point called ``setup``. This setup must be a plain Python function (not a coroutine). It takes a single parameter -- the :class:`~.commands.Bot` that loads the extension.
+An extension at its core is a python file with an entry point called ``setup``. This setup function must be a Python coroutine. It takes a single parameter -- the :class:`~.commands.Bot` that loads the extension.
 
 An example extension looks like this: