[DOC] Miss underline before cmd in example code

[NatLee]

Summary

I spent one hour for trying the code.
Finally, I find there is a mistake. : (

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

• Add an underline.

Acknowledgment

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

[codecov]

Codecov Report

Merging #3107 into master will increase coverage by 0.63%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3107      +/-   ##
==========================================
+ Coverage   67.23%   67.86%   +0.63%     
==========================================
  Files         294      295       +1     
  Lines       39245    39263      +18     
  Branches     5162     5172      +10     
==========================================
+ Hits        26386    26646     +260     
+ Misses      12185    11912     -273     
- Partials      674      705      +31

Flag	Coverage Δ
#smoketests	51.23% <ø> (ø)	⬆️
#unittests	65.09% <ø> (+0.87%)	⬆️

Impacted Files	Coverage Δ
nipype/pipeline/plugins/legacymultiproc.py	62.81% <0%> (-0.51%)	⬇️
nipype/testing/__init__.py	88.88% <0%> (ø)
nipype/interfaces/spm/preprocess.py	56.93% <0%> (+0.32%)	⬆️
nipype/utils/filemanip.py	77.59% <0%> (+0.47%)	⬆️
nipype/interfaces/freesurfer/preprocess.py	66.04% <0%> (+0.98%)	⬆️
nipype/interfaces/freesurfer/model.py	64.19% <0%> (+1.12%)	⬆️
nipype/interfaces/spm/base.py	87% <0%> (+2%)	⬆️
nipype/interfaces/freesurfer/utils.py	62.88% <0%> (+2.13%)	⬆️
nipype/interfaces/io.py	59.67% <0%> (+6.11%)	⬆️
nipype/interfaces/dcm2nii.py	64.78% <0%> (+15.96%)	⬆️
... and 3 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0225a45...f4ca8f0. Read the comment docs.

[mgxd]

Thank you!
Do you want to add yourself to the .zenodo.json file?

[NatLee]

Oh! Thank you!
I added the modifed on the other request : )
#3108

[mgxd]

@NatLee you can add the change to this pull request by altering this branch's zenodo file:

https://github.com/NatLee/nipype/blob/patch-1/.zenodo.json

[NatLee]

I added.
Thank you : )