docs: fix DEP003 example by explicitly adding certifi

[huisman]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added or modified a feature, documentation in docs is updated

Description of changes
The example in the documentation for DEP003: transitive dependencies mentions explicitly adding certifi to the dependencies as the solution.
However (and confusingly) in the toml code example the package httpcore is added.
This PR makes sure the example toml code matches the written instructions by replacing httpcore in the example with certifi.

[mkniewallner]

Nice catch, thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.0%. Comparing base (4f20d7f) to head (c562502).

Additional details and impacted files

@@          Coverage Diff          @@
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  Coverage   93.0%   93.0%           
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  Files         37      37           
  Lines        961     961           
  Branches     173     173           
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  Hits         894     894           
  Misses        51      51           
  Partials      16      16           

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