Fixing bad regex pattern in instruments.py

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 33.33%. Comparing base (141975a) to head (a82be37). Report is 1 commits behind head on main.

Files	Patch %	Lines
mkidcore/instruments.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11 +/- ## ======================================= Coverage 33.33% 33.33% ======================================= Files 11 11 Lines 1647 1647 ======================================= Hits 549 549 Misses 1098 1098 ```

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