Fix formatting of pip install commmand in docs

[tvainika]

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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	Coverage Δ
cext	91.61% <ø> (-0.02%)	⬇️
integration	94.56% <ø> (-0.02%)	⬇️
purepy	94.41% <ø> (-0.03%)	⬇️
unit	44.91% <ø> (ø)

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