Fix doctest for MDAnalysis.analysis.psa

[SorenKyhl]

Towards #3925 for MDAnalysis.analysis.psa

I'm submitting this tiny PR to check if it is acceptable before committing further down the error list- a 'micro' contribution to the 'mega-Issue.'

Changes made in this Pull Request:

• Update the docstrings in MDAnalysis.analysis.psa so that the doctests pass for this particular file.

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[github-actions]

Hello there first time contributor! Welcome to the MDAnalysis community! We ask that all contributors abide by our Code of Conduct and that first time contributors introduce themselves on the developer mailing list so we can get to know you. You can learn more about participating here. Please also add yourself to package/AUTHORS as part of this PR.

[codecov]

Codecov Report

Base: 93.53% // Head: 93.53% // No change to project coverage 👍

Coverage data is based on head (1442083) compared to base (7a7bcdc).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4038   +/-   ##
========================================
  Coverage    93.53%   93.53%           
========================================
  Files          191      191           
  Lines        25063    25063           
  Branches      3548     3548           
========================================
  Hits         23443    23443           
  Misses        1099     1099           
  Partials       521      521           

Impacted Files	Coverage Δ
package/MDAnalysis/analysis/psa.py	84.21% <ø> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[SorenKyhl]

Somehow I didn't see @ooprathamm's PR (#4035) resolving doctests in this very file. It looks like he has it covered! My apologies for the redundant PR!