Fix incorrect error message for opto stimulus site

[rly]

Motivation

The error message is incorrect.

tests failing

[rly]

Thanks for updating the test! You'll have to re-approve.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.83%. Comparing base (c453b68) to head (77065e3).
Report is 2 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #524      +/-   ##
==========================================
+ Coverage   82.77%   86.83%   +4.05%     
==========================================
  Files          47       47              
  Lines        1504     1504              
==========================================
+ Hits         1245     1306      +61     
+ Misses        259      198      -61     

Files with missing lines	Coverage Δ
src/nwbinspector/checks/_ogen.py	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes