docs(backport): Correct fixed_params type to tuple or list

[matthewfeickert]

Description

• Backport PR docs: Correct fixed_params type to tuple or list #2420
• fixed_params is an iterable that is of type tuple or list. Though operations on it may support tensor types, not all operations will necessarily be supported and so the docs should not list it as being a tensor.

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* Backport PR https://github.com/scikit-hep/pyhf/pull/2420
* `fixed_params` is an iterable that is of type tuple or list. Though
  operations on it may support tensor types, not all operations will
  necessarily be supported and so the docs should not list it as being
  a tensor.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (release/v0.7.x@6bf1f4a). Click here to learn what that means.

Additional details and impacted files

@@                Coverage Diff                @@
##             release/v0.7.x    #2428   +/-   ##
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  Coverage                  ?   98.21%           
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  Files                     ?       69           
  Lines                     ?     4543           
  Branches                  ?      803           
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  Hits                      ?     4462           
  Misses                    ?       47           
  Partials                  ?       34           

Flag	Coverage Δ
contrib	27.53% <0.00%> (?)
doctest	60.99% <0.00%> (?)
unittests-3.10	96.25% <0.00%> (?)
unittests-3.11	96.25% <0.00%> (?)
unittests-3.7	96.23% <0.00%> (?)
unittests-3.8	96.27% <0.00%> (?)
unittests-3.9	96.30% <0.00%> (?)

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