Fix a typo in documentation

docs/index.txt

@@ -18,7 +18,7 @@ tagging to your project easy and fun.
    Since version 0.12.0 taggit uses Django migrations by default. South users
    Since version 0.12.0 taggit uses Django migrations by default. South users
    have to adjust their settings::
    have to adjust their settings::
 
 
-      SOUTH_MIGRATIONS_MODULES = {
+      SOUTH_MIGRATION_MODULES = {
           'taggit': 'taggit.south_migrations',
           'taggit': 'taggit.south_migrations',
       }
       }