[issue36168] DOC: Fix capitalization in string.rst

[Roundup Robot]

Change by Roundup Robot <devnull at psf.upfronthosting.co.za>:


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keywords: +patch
pull_requests: +14302
stage: needs patch -> patch review
pull_request: https://github.com/python/cpython/pull/14485

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