`call_parse` example missing default value for `upper`

It seems to me that `upper` should be given a default value in the code snippet in https://fastcore.fast.ai/script.html#example, given that it is intended to be an optional parameter.

Open the call_parse example at https://fastcore.fast.ai/script.html#example and inspect how the optional upper parameter is shown. Update the snippet so upper has a default value, then verify the example still communicates that the parameter is optional.

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