Fix math erros in `models.dbi.double_bracket` docstrings

[renatomello]

Closes #1388. However, there are several methods in those classes that have no description of their args in their respective dosctrings. Someone with more knowledge of the classes should fix that too.

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 99.99%. Comparing base (d8cc501) to head (560c4de).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1394 +/- ## ======================================= Coverage 99.99% 99.99% ======================================= Files 78 78 Lines 11192 11192 ======================================= Hits 11191 11191 Misses 1 1 ``` | [Flag](https://app.codecov.io/gh/qiboteam/qibo/pull/1394/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/1394/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | `99.99% <100.00%> (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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