Fix link in install instructions

[glatterf42]

As pointed out by Marek, the link to GitHub in footnote 2 of the install instructions was broken.

How to review

• Read the diff and note that the CI checks all pass.
• Build the documentation, look at the installation page, try to follow the link in 'Anaconda -> Step 4 -> Footnote 2'

PR checklist

• Continuous integration checks all ✅
  Except for macos-latest-py3.7 per Track/work around actions/setup-python#682 ixmp#484.
• Add, expand, or update documentation.

[codecov]

Codecov Report

Merging #724 (5e6836a) into main (3a8910b) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #724   +/-   ##
=====================================
  Coverage   94.4%   94.4%           
=====================================
  Files         43      43           
  Lines       3448    3448           
=====================================
  Hits        3257    3257           
  Misses       191     191           

[glatterf42]

This PR could also be a vehicle for the suggested fix for actions/setup-python#682 (comment) if we don't want to wait for the changes to reach us naturally.