Fixed typo in error message in django.contrib.sitemaps.

class FlatPageSitemap(Sitemap):

    def items(self):

        if not django_apps.is_installed('django.contrib.sites'):

            raise ImproperlyConfigured("ping_google requires django.contrib.sites, which isn't installed.")

            raise ImproperlyConfigured("FlatPageSitemap requires django.contrib.sites, which isn't installed.")

        Site = django_apps.get_model('sites.Site')

        current_site = Site.objects.get_current()

        return current_site.flatpage_set.filter(registration_required=False)