Fixed typo in docs/ref/contrib/sites.txt

The :setting:`SITE_ID` setting specifies the database ID of the

:class:`~django.contrib.sites.models.Site` object associated with that

particular settings file. It the setting is omitted, the

particular settings file. If the setting is omitted, the

:func:`~django.contrib.sites.shortcuts.get_current_site` function will

try to get the current site by comparing the

:attr:`~django.contrib.sites.models.Site.domain` with the host name from