Fix class hierarchy elements taking too much vertical space

[tristanlatr]

Just a small css change.

[codecov[bot]]

Codecov Report

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

Project coverage is 92.61%. Comparing base (7e0a09f) to head (758d1b9).

:exclamation: Current head 758d1b9 differs from pull request most recent head 9de3d82. Consider uploading reports for the commit 9de3d82 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #785 +/- ## ========================================== - Coverage 92.65% 92.61% -0.04% ========================================== Files 47 47 Lines 8373 8373 Branches 1844 1844 ========================================== - Hits 7758 7755 -3 - Misses 354 356 +2 - Partials 261 262 +1 ```

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[github-actions[bot]]

According to pydoctor_primer, this change doesn't affect pydoctor warnings on a corpus of open source code. ✅

[github-actions[bot]]

According to pydoctor_primer, this change doesn't affect pydoctor warnings on a corpus of open source code. ✅