Fix math rendering in some docsstrings of the `quantum_info` module

[renatomello]

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 97.10%. Comparing base (218f298) to head (5a7fe1e). Report is 12 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1441 +/- ## ======================================= Coverage 97.10% 97.10% ======================================= Files 81 81 Lines 11653 11653 ======================================= Hits 11316 11316 Misses 337 337 ``` | [Flag](https://app.codecov.io/gh/qiboteam/qibo/pull/1441/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/1441/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | `97.10% <ø> (ø)` | | 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.

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[renatomello]

Thanks! Is the choice of the notation | v ) common? Just curiosity :)

yes, either that or double kets like $$|A\rangle\rangle$$

[alecandido]

yes, either that or double kets like | A ⟩⟩

I've also seen the double kets somewhere in the docs. Should we keep just one or the other?

[renatomello]

yes, either that or double kets like | A ⟩⟩

I've also seen the double kets somewhere in the docs. Should we keep just one or the other?

Done

[alecandido]

Done

Thanks @renatomello!