[docs] unnecessary coma in code-example

Hi, sorry to bother, i think i found an unecessary comma at: http://docs.python.org/py3k/tutorial/datastructures.html#nested-list-compreh... in the first code-example after the last element in the list

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matrix = [ ... [1, 2, 3, 4], ... [5, 6, 7, 8], ... [9, 10, 11, 12], ... ]

Thanks for providing such a great website and the superb organized content! Regards Martin