Update config-local-settings.rst

The docs are incorrect compared to the code:

```
$ grep -nri REMOTE_USER_AUTHENTICATION .
./webapp/graphite/local_settings.py.example:131:#USE_REMOTE_USER_AUTHENTICATION = True
./webapp/graphite/settings.py:94:USE_REMOTE_USER_AUTHENTICATION = False
./webapp/graphite/settings.py:209:if USE_REMOTE_USER_AUTHENTICATION:
./docs/config-local-settings.rst:217:REMOTE_USER_AUTHENTICATION
```
Merge pull request #534 from jib/remote-auth-doc-fix

Update config-local-settings.rst