Fix documentation of TrioEventLoop.run_async()

[ntamas]

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Description:

This PR fixes the documentation of TrioEventLoop.run_async(), which incorrectly suggests that the constructor of TrioEventLoop() has a nursery argument (which it doesn't have).

The new snippet in the docstring now accurately reflects how TrioEventLoop should be used in an existing Trio event loop; copied straight from one of my projects.

[coveralls]

Coverage remained the same at 77.868% when pulling 59b2942 on collmot:fix/doc-fix-TrioEventLoop-run-async into 4c739b6 on urwid:master.

[tonycpsu]

LGTM, thanks.