bpo-30709: Fixed bad getter example from Doc/Descriptor#Properties

[anqur]

• I recreated the branch that is current even with master
• Previous conversation: bpo-30709: bad getter from Descriptor#Properties example #2288
• About the commit,
  • The purpose of obj in the example is implicit
  • As an argument the obj will cause errors when using Cell('b10').value because it expects more arguments, but 1 given

[mention-bot]

@anqurvanillapy, thanks for your PR! By analyzing the history of the files in this pull request, we identified @birkenfeld, @serhiy-storchaka and @rhettinger to be potential reviewers.

[the-knights-who-say-ni]

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[Mariatta]

Thanks @anqurvanillapy and congrats on your first contribution to CPython 🎉