Fix formatting of pip install commmand in docs

Changes

Fixes documentation formatting of pip install commands.

Checklist

[ ] I think the code is well written
[ ] Unit tests for the changes exist
[ ] Documentation reflects the changes
[ ] Add a new news fragment into the CHANGES folder
- name it <issue_id>.<type> (e.g. 588.bugfix)
- if you don't have an issue_id change it to the pr id after creating the PR
- ensure type is one of the following:
- .feature: Signifying a new feature.
- .bugfix: Signifying a bug fix.
- .doc: Signifying a documentation improvement.
- .removal: Signifying a deprecation or removal of public API.
- .misc: A ticket has been closed, but it is not of interest to users.
- Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

Codecov Report

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

Comparison is base (85f758e) 94.92% compared to head (95ba2ee) 94.91%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #956 +/- ## ========================================== - Coverage 94.92% 94.91% -0.02% ========================================== Files 106 106 Lines 16318 16318 Branches 2615 2615 ========================================== - Hits 15490 15488 -2 - Misses 552 554 +2 Partials 276 276 ``` | [Flag](https://app.codecov.io/gh/aio-libs/aiokafka/pull/956/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | Coverage Δ | | |---|---|---| | [cext](https://app.codecov.io/gh/aio-libs/aiokafka/pull/956/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `91.61% <ø> (-0.02%)` | :arrow_down: | | [integration](https://app.codecov.io/gh/aio-libs/aiokafka/pull/956/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `94.56% <ø> (-0.02%)` | :arrow_down: | | [purepy](https://app.codecov.io/gh/aio-libs/aiokafka/pull/956/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `94.41% <ø> (-0.03%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/aio-libs/aiokafka/pull/956/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `44.91% <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=aio-libs#carryforward-flags-in-the-pull-request-comment) to find out more.

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aio-libs / aiokafka