docs: Improve async.md

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (df30923) to head (3176025). Report is 4 commits behind head on dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #502 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 312 312 Lines 8960 8960 ========================================= Hits 8960 8960 ``` | [Flag](https://app.codecov.io/gh/Mirascope/mirascope/pull/502/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Mirascope) | Coverage Δ | | |---|---|---| | [tests](https://app.codecov.io/gh/Mirascope/mirascope/pull/502/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Mirascope) | `100.00% <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=Mirascope#carryforward-flags-in-the-pull-request-comment) to find out more.

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