Adds missing f to f string for printing info about variable

[dulte]

Change Summary

A f string in Variables was broken. This fixes it

Checklist

• Start with a draft-PR
• The PR title is a good summary of the changes
• PR is set to AeroTools and a tentative milestone
• Documentation reflects the changes where applicable
• Tests for the changes exist where applicable
• Tests pass locally
• Tests pass on CI
• At least 1 reviewer is selected
• Make PR ready to review

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.31%. Comparing base (ac80b76) to head (1fd6170).
Report is 485 commits behind head on main-dev.

Files with missing lines	Patch %	Lines
pyaerocom/variable.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           main-dev    #1198      +/-   ##
============================================
- Coverage     79.31%   79.31%   -0.01%     
============================================
  Files           131      131              
  Lines         20230    20229       -1     
============================================
- Hits          16046    16045       -1     
  Misses         4184     4184              

Flag	Coverage Δ
unittests	79.31% <0.00%> (-0.01%)	⬇️

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