[issue36168] DOC: Fix capitalization in string.rst

[Readul Hasan Chayan]

Readul Hasan Chayan <heemayl at gmail.com> added the comment:

If no one has any problem I would like to work on this. This will be my first contribution if it happens.

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nosy: +Readul Hasan Chayan

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36168>
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