Minor docs example fix

Dates are smaller when they happen before.

docs/api-guide/serializers.md

@@ -161,7 +161,7 @@ To do any other validation that requires access to multiple fields, add a method
             """
             Check that the start is before the stop.
             """
-            if attrs['start'] < attrs['finish']:
+            if attrs['start'] > attrs['finish']:
                 raise serializers.ValidationError("finish must occur after start")
             return attrs