docs: correct references to __isub__ etc

[davidhewitt]

Looking at the sub-slots table in https://docs.python.org/3.9/c-api/typeobj.html#sub-slots, it looked to be that the inplace methods such as __isub__ were incorrectly referring to the non-inplace methods such as __sub__? This PR corrects those typos, if I am correct.

[the-knights-who-say-ni]

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[miss-islington]

Thanks @davidhewitt for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9, 3.10.
🐍🍒⛏🤖

[miss-islington]

Sorry, @davidhewitt and @serhiy-storchaka, I could not cleanly backport this to 3.9 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 800bd01b6d365156403c3dde61765dc5fdbf64be 3.9

[bedevere-bot]

GH-28383 is a backport of this pull request to the 3.10 branch.

[bedevere-bot]

GH-28415 is a backport of this pull request to the 3.9 branch.