Fix definition of fidelity in quantum_info.metrics

[renatomello]

Two different definitions of fidelity were being mixed, one with $\text{Tr}$ and another with $\text{Tr}^{2}$. This PR fixes it.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.93%. Comparing base (fee545b) to head (d254f8e).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1463      +/-   ##
==========================================
- Coverage   99.94%   99.93%   -0.02%     
==========================================
  Files          81       81              
  Lines       11737    11737              
==========================================
- Hits        11731    11729       -2     
- Misses          6        8       +2     

Flag	Coverage Δ
unittests	99.93% <100.00%> (-0.02%)	⬇️

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