hotfix: add f-string identifier to warning string

[jpdehollain]

One of the warnings in data_set_in_memory.py was missing the f in front of the string.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 67.18%. Comparing base (ec7bad5) to head (69192f9).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6253 +/- ## ======================================= Coverage 67.18% 67.18% ======================================= Files 352 352 Lines 32140 32140 ======================================= Hits 21594 21594 Misses 10546 10546 ```

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