fix: updated the google docstring url

[kachida]

Updated Google Docstring Format link since currently it points to a 404 page.

Fixes https://github.com/python-trio/trio/issues/2546

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.64%. Comparing base (48f8293) to head (f5d6c89).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2967 +/- ## ======================================= Coverage 99.64% 99.64% ======================================= Files 117 117 Lines 17634 17634 Branches 3172 3172 ======================================= Hits 17572 17572 Misses 43 43 Partials 19 19 ```

[jakkdl]

Though maybe there might be a stable version of the docs or something, but seems like master is the only option so nothing to consider. Thanks!

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