[FIX] change fsl interface randomise --f_only to --fonly for #3322

[henrymj]

Summary

Fixes #3322.

List of changes proposed in this PR (pull-request)

• corrects typo in fsl randomise interface for running only f-tests.

Acknowledgment

• [ x] (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[codecov]

Codecov Report

Merging #3325 (fe4cf08) into master (6c06030) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3325   +/-   ##
=======================================
  Coverage   65.00%   65.00%           
=======================================
  Files         302      302           
  Lines       40027    40027           
  Branches     5292     5292           
=======================================
  Hits        26021    26021           
  Misses      12934    12934           
  Partials     1072     1072           

Flag	Coverage Δ
unittests	64.70% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
nipype/interfaces/fsl/model.py	55.14% <100.00%> (ø)

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[effigies]

I can confirm that --fonly is also correct for FSL 5.0.9. Thanks for the contribution!