Rename NOSE_WITH_DOCTEST => NOSE_WITH_DOCTESTS in documentation.

The docs previously said you DOCTEST but should have said DOCTESTS (note plural).

The behaviour is defined in [this source line](https://github.com/nose-devs/nose/blob/release_1.3.0/nose/plugins/base.py#L83) and the fact that the plugin's name is ``doctests.py`` rather than ``doctest.py``