updated incorrect information in readme file.

The Readme has the wrong attributes for the Meta class, so I fixed them.

According to https://github.com/nephila/django-meta/blob/develop/meta/views.py#L19-20 there is no ``meta_extras`` or ``custom_meta_extras`` attributes on Meta

I switched ``meta_extras`` with ``extra_props`` and ``custom_meta_extras`` with ``extra_custom_props``