Fix variable name in classifier fitting

[MatteoRobbiati]

Small fix to che classifier routine.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.
• Compatibility with Qibo modules (Please edit this section if the current pull request is not compatible with the following branches).
  • Qibo: master
  • Qibolab: main
  • Qibolab_platforms_qrc: main

[codecov]

Codecov Report

Merging #553 (b381353) into main (cea9d85) will increase coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #553      +/-   ##
==========================================
+ Coverage   95.90%   95.95%   +0.05%     
==========================================
  Files          83       83              
  Lines        5740     5740              
==========================================
+ Hits         5505     5508       +3     
+ Misses        235      232       -3     

Flag	Coverage Δ
unittests	95.95% <0.00%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/qibocal/fitting/classifier/run.py	96.69% <0.00%> (ø)

... and 1 file with indirect coverage changes

[andrea-pasquale]

@MatteoRobbiati if you merge main here I believe that tests should pass

[andrea-pasquale]

@Edoardo-Pedicillo could you please check if coverage could be fixed?

[Edoardo-Pedicillo]

Thank @MatteoRobbiati lgtm