[CMAKE] fix a typo in FindMiniUPnPC
[CMAKE] fix a typo in FindMiniUPnPC

Summary: as per title

Test Plan:
```
cmake .. -GNinja
ninja && ninja check
```

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12085

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