README : Mopidy version on pip is now >= 2.3.3

As written in the title, the Mopidy version on pip is now >= 2.3.3

Because of a bug in recent mopidy versions <https://github.com/mopidy/mopidy/issues/1528>_, if you face any playback issues, you should use at least Mopidy 2.2.3 which is currently unreleased, or install the development version with pip install --user git+https://github.com/mopidy/mopidy.git.

This line seeems now unnecessary (I haven't check on debian apt based installation)