Fixed #12517 -- Corrected get_prep_lookup example in custom field docs. Thanks to django@pressure.net.nz for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13213 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/howto/custom-model-fields.txt

@@ -582,7 +582,7 @@ accepted lookup types to ``exact`` and ``in``::
         def get_prep_lookup(self, lookup_type, value):
             # We only handle 'exact' and 'in'. All others are errors.
             if lookup_type == 'exact':
-                return [self.get_prep_value(value)]
+                return self.get_prep_value(value)
             elif lookup_type == 'in':
                 return [self.get_prep_value(v) for v in value]
             else: