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:

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

[codecov[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/qiboteam/qibo/pull/1463/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/qiboteam/qibo/pull/1463/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | `99.93% <100.00%> (-0.02%)` | :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=qiboteam#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.