Correct the installation instructions for Windows

[alexpdp7]

See rendering on https://github.com/alexpdp7/OCRmyPDF/blob/main/docs/installation.rst#installing-on-windows

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.16%. Comparing base (e4274a9) to head (acea952).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1485   +/-   ##
=======================================
  Coverage   90.16%   90.16%           
=======================================
  Files          95       95           
  Lines        7128     7128           
  Branches      729      729           
=======================================
  Hits         6427     6427           
  Misses        496      496           
  Partials      205      205           

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