[1.7.x] Fixed typo in error message in django.contrib.sitemaps.

Backport of d81a9c50 from master

django/contrib/sitemaps/__init__.py

@@ -117,7 +117,7 @@ class Sitemap(object):
 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)