Typo in entrypoints

In `setup.py`, the `pytest11` entrypoint is called _`pyest_osxnotify`_ -- note the missing "**t**".

You might also consider naming the entrypoint just "_osxnotify_". The one place that I know this is used for this plugin is disabling it from the command-line: Currently one has to use `-p no:pyest_osxnotify`; it would be more conventional to be able to use `-p no:osxnotify`.
