Fix Logout Tag in Docs ({% url auth_logout %})

[dbrgn]

After some playing around I found out that you need to change the {% url auth_logout %} tag in your README to {% url social_logout %}.

[flashingpumpkin]

`{% url social_logout %}` is just a wrapper around the built in `{% url auth_logout %}`, but I'll change

[dbrgn]

auth_logout did not work for me, social_logout did. Whatever the problem was, thanks for the change.