[DOC,FIX] Doxygen warning

[eseiler]

https://cdash.seqan.de/test/24113657

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Name	Status	Preview	Updated
seqan3	✅ Ready (Inspect)	Visit Preview	Dec 14, 2022 at 1:13PM (UTC)

[codecov[bot]]

Codecov Report

Base: 98.22% // Head: 98.22% // No change to project coverage :thumbsup:

Coverage data is based on head (adb6408) compared to base (3c74702). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3115 +/- ## ======================================= Coverage 98.22% 98.22% ======================================= Files 275 275 Lines 12192 12192 ======================================= Hits 11975 11975 Misses 217 217 ``` | [Impacted Files](https://codecov.io/gh/seqan/seqan3/pull/3115?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=seqan) | Coverage Δ | | |---|---|---| | [include/seqan3/alphabet/structure/dssp9.hpp](https://codecov.io/gh/seqan/seqan3/pull/3115/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=seqan#diff-aW5jbHVkZS9zZXFhbjMvYWxwaGFiZXQvc3RydWN0dXJlL2Rzc3A5LmhwcA==) | `100.00% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=seqan). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=seqan)

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