Fix typo labeling PyUnicodeType's tp_basicsize field as "tp_size"

[mr-nfamous]

Fixed the typo on line 15065 documenting PyUnicodeType.tp_basicsize as "tp_size" and realigned the comment labels.

[the-knights-who-say-ni]

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[zhangyangyu]

Thanks @mr-nfamous . I merged your PR as an improvement. :-) Actually there are still some tp_size in other modules' comments. But I would suggest not to churn the codes in other modules since it doesn't do any harm and seems developers could identify what they mean. Thanks for your contribution. :-)