DOCS: Correct variation_type data type in parametric setup documentation

[jvela018]

Description

The functions add() and add_variation() within the ParametricSetup and SetupParam classes expect to be overloaded with variation_type variable of type str. However, initially it was int or float. This generated a warning when passing this argument as shown in the image below

The change in the docstring removes these warnings

Issue linked

No issue linked to this

Checklist

• I have tested my changes locally.
• I have added necessary documentation or updated existing documentation.
• I have followed the coding style guidelines of this project.
• I have added appropriate tests (unit, integration, system).
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved by the PR if any.
• I have agreed with the Contributor License Agreement (CLA).

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.68%. Comparing base (3a4b738) to head (c224ab3).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5483   +/-   ##
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  Coverage   84.68%   84.68%           
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  Files         143      143           
  Lines       60110    60110           
=======================================
+ Hits        50903    50905    +2     
+ Misses       9207     9205    -2     

[Samuelopez-ansys]

LGTM