[DOC] Fixed Developer Setup Link in install.rst

[RaunakJalan]

Summary

Changed the link for developer setup in install.rst to make it working. Previously showing Error 404.
Now the link is working and redirecting to appropriate page.

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

• Change link of developer setup to <../devel/testing_nipype.rst> instead of <../devel/testing_nipype.html>

Acknowledgment

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

[codecov]

Codecov Report

Merging #3330 (dc57459) into master (89d828c) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3330      +/-   ##
==========================================
+ Coverage   65.05%   65.09%   +0.04%     
==========================================
  Files         302      302              
  Lines       40027    40027              
  Branches     5292     5292              
==========================================
+ Hits        26038    26055      +17     
+ Misses      12916    12904      -12     
+ Partials     1073     1068       -5     

Flag	Coverage Δ
unittests	65.01% <ø> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
nipype/pipeline/plugins/base.py	59.72% <0.00%> (+1.64%)	⬆️
nipype/pipeline/plugins/legacymultiproc.py	71.07% <0.00%> (+5.39%)	⬆️

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[effigies]

Great, thanks! Would you like to add yourself to the .zenodo.json file?

[RaunakJalan]

Yeah sure @effigies. I would like to add myself to it. Looking to make more contributions in future.

[effigies]

Sounds great! Just add your information after this entry: https://github.com/RaunakJalan/nipype/blob/doc_issue/.zenodo.json#L796-L800