Updated helium nlte page in docs with in-text citation

[erinvisser]

📝 Description

Type: 📝 documentation

The Helium NLTE docs page had a line referring to information "in an upcoming paper," but that paper came out in 2017. I added the paper to the list of references and replaced "in an upcoming paper" with an in-text citation.

📌 Resources

🚦 Testing

How did you test these changes?

• Testing pipeline
• Other method (describe)
• My changes can't be tested (explain why)

☑️ Checklist

• I requested two reviewers for this pull request
• I updated the documentation according to my changes
• I built the documentation by applying the build_docs label

Note: If you are not allowed to perform any of these actions, ping (@) a contributor.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.51%. Comparing base (763f571) to head (a026895).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2574   +/-   ##
=======================================
  Coverage   67.51%   67.51%           
=======================================
  Files         170      170           
  Lines       14230    14230           
=======================================
  Hits         9608     9608           
  Misses       4622     4622           

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