FIX: Missed np.savetxt bstring

[mgxd]

Popped up in tests - remaining from b41fb1c

[codecov]

Codecov Report

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

Project coverage is 73.04%. Comparing base (37546c7) to head (292e35c).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
nipype/algorithms/confounds.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3704      +/-   ##
==========================================
- Coverage   73.13%   73.04%   -0.09%     
==========================================
  Files        1278     1278              
  Lines       59356    59356              
==========================================
- Hits        43411    43359      -52     
- Misses      15945    15997      +52     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[effigies]

Should we release with this?