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

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

[codecov[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/metno/pyaerocom/pull/1198/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/metno/pyaerocom/pull/1198/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno) | `79.31% <0.00%> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno#carryforward-flags-in-the-pull-request-comment) to find out more.

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