Fix the download links for macos

[Cadair]

The download page linked to installers with MacOSX in the filename, but then told people to execute the script on macos by running bash Miniforge3-$(uname)[...] which returns Darwin not MacOSX. This fix changes the URL to match the script.

Fixes conda-forge/miniforge#808

PR Checklist:

• note any issues closed by this PR with closing keywords
• if you are adding a new page under docs/ or community/, you have added it to the sidebar in the corresponding _sidebar.json file
• put any other relevant information below

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[hmaarrfk]

Although I don't like the fact that we have both download links, there are convinent for instructions.