DOC: Fix naming of motion parameters (roll/yaw swapped)

[abelalez]

I've modified the docstring in the function normalize_mc_params, after discussing with @satra.
There was a mismatch in the description of the parameters ry and rz.
ry -> roll, rz -> yaw

Summary

Fixes # .

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

Acknowledgment

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

[codecov-io]

Codecov Report

Merging #2696 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2696      +/-   ##
==========================================
+ Coverage    67.6%   67.61%   +<.01%     
==========================================
  Files         340      340              
  Lines       43144    43144              
  Branches     5349     5349              
==========================================
+ Hits        29169    29173       +4     
+ Misses      13273    13269       -4     
  Partials      702      702

Flag	Coverage Δ
#smoketests	50.58% <ø> (ø)	⬆️
#unittests	65.1% <ø> (+0.03%)	⬆️

Impacted Files	Coverage Δ
nipype/utils/misc.py	64.15% <ø> (ø)	⬆️
nipype/pipeline/plugins/legacymultiproc.py	63.69% <0%> (+2.38%)	⬆️

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[effigies]

Thanks!