Corrected bc20m variable description

[j-a-foster]

Description

Corrects description for variable bc20m.

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.98%. Comparing base (4885172) to head (03eec19).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4641      +/-   ##
==========================================
+ Coverage   49.92%   49.98%   +0.06%     
==========================================
  Files         151      151              
  Lines       30236    30450     +214     
==========================================
+ Hits        15094    15220     +126     
- Misses      15142    15230      +88     

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