corrected level1design EV parameter substitution

[TheChymera]

and corrected docstring

[codecov-io]

Codecov Report

Merging #1953 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #1953      +/-   ##
=========================================
- Coverage   72.51%   72.5%   -0.01%     
=========================================
  Files        1063    1063              
  Lines       54209   54209              
  Branches     7825    7825              
=========================================
- Hits        39307   39305       -2     
- Misses      13680   13682       +2     
  Partials     1222    1222

Flag	Coverage Δ
#smoketests	72.5% <ø> (-0.01%)	⬇️
#unittests	70.05% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/fsl/model.py	80.57% <ø> (ø)	⬆️
nipype/interfaces/base.py	84.66% <0%> (-0.19%)	⬇️

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