[doc] fix RDF equation (and hole link)

[orbeckst]

Corrects error in the RDF equation as noted by @rodpollet #4437 (reply in thread)

Changes made in this Pull Request:

• fix RDF g_ab equation
• fix missing reST link in the (deprecated) hole2 module

PR Checklist

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

Developers certificate of origin

• I certify that this contribution is covered by the LGPLv2.1+ license as defined in our LICENSE and adheres to the Developer Certificate of Origin.

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📚 Documentation preview 📚: https://mdanalysis--4591.org.readthedocs.build/en/4591/

[pep8speaks]

Hello @orbeckst! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file package/MDAnalysis/analysis/rdf.py:

Line 35:80: E501 line too long (82 > 79 characters)

[github-actions]

Linter Bot Results:

Hi @orbeckst! Thanks for making this PR. We linted your code and found the following:

There are currently no issues detected! 🎉

[hmacdope]

LGTM @orbeckst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.62%. Comparing base (45fb664) to head (61d05a9).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4591      +/-   ##
===========================================
- Coverage    93.64%   93.62%   -0.02%     
===========================================
  Files          168      180      +12     
  Lines        21254    22333    +1079     
  Branches      3918     3918              
===========================================
+ Hits         19904    20910    +1006     
- Misses         892      965      +73     
  Partials       458      458              

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