Friendly 57f9d7a6ce doc: markdown heading
The previous markdown was not valid and yielded the following warning:

```sh
make html
Running Sphinx v5.3.0
WARNING: Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).
making output directory... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 34 source files that are out of date
updating environment: [new config] 34 added, 0 changed, 0 removed
reading sources... [100%] virtual_machines
hackrf/docs/source/list_of_hardware_revisions.rst:36: WARNING: Title underline too short.

HackRF One r10
~~~~~~~~~~~~~
hackrf/docs/source/list_of_hardware_revisions.rst:36: WARNING: Title underline too short.

HackRF One r10
~~~~~~~~~~~~~
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] virtual_machines
generating indices... genindex done
writing additional pages... search done
copying images... [100%] ../images/noisereducingcablescreenshot.jpeg
copying static files... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded, 3 warnings.

The HTML pages are in build/html.
```

Congratulations on double digits.
2024-03-03 19:24:18 -06:00
..
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2021-07-02 11:52:22 -06:00