fixed incorrect duecredit environment variable in documentation

[ianmkenney]

Changes made in this Pull Request:

• Changed example duecredit environment variable from
  DUECREDIT-ENABLE (invalid) to DUECREDIT_ENABLE

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.

---

📚 Documentation preview 📚: https://mdanalysis--4226.org.readthedocs.build/en/4226/

[github-actions]

Linter Bot Results:

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

There are currently no issues detected! 🎉

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01% ⚠️

Comparison is base (2bf55dd) 93.62% compared to head (d733ed2) 93.62%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4226      +/-   ##
===========================================
- Coverage    93.62%   93.62%   -0.01%     
===========================================
  Files          193      193              
  Lines        25295    25294       -1     
  Branches      4063     4063              
===========================================
- Hits         23683    23682       -1     
  Misses        1096     1096              
  Partials       516      516              

see 1 file with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[orbeckst]

thanks for fixing