Fixed typo in parameter name doc for BaseCommand.check().

    potential problems. Serious problems are raised as a :class:`CommandError`;

    warnings are output to stderr; minor notifications are output to stdout.

    If ``apps`` and ``tags`` are both None, all system checks are performed.

    ``tags`` can be a list of check tags, like ``compatibility`` or ``models``.

    If ``app_configs`` and ``tags`` are both ``None``, all system checks are

    performed. ``tags`` can be a list of check tags, like ``compatibility`` or

    ``models``.

.. method:: BaseCommand.validate(app=None, display_num_errors=False)