Fixed #18528 -- Fixed custom field value_to_string example

Thanks anuraguniyal for the report.

docs/howto/custom-model-fields.txt

@@ -666,7 +666,7 @@ data storage anyway, we can reuse some existing conversion code::
 
         def value_to_string(self, obj):
             value = self._get_val_from_obj(obj)
-            return self.get_db_prep_value(value)
+            return self.get_prep_value(value)
 
 Some general advice
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