Fix shape check

[tsalo]

Closes none. Fixes a bug I introduced in #67.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 30.87%. Comparing base (66b4c66) to head (1fd30dd).
Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
src/fmripost_aroma/interfaces/confounds.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #68   +/-   ##
=======================================
  Coverage   30.87%   30.87%           
=======================================
  Files          26       26           
  Lines        2329     2329           
  Branches      365      365           
=======================================
  Hits          719      719           
  Misses       1589     1589           
  Partials       21       21           

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