Standardize macOS naming in README.md

[amorehead]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[hmaarrfk]

Thank you! This is a welcome change, but you seem to only be adding "more variants".

[amorehead]

Ah, good point. I'll fix that now.

[hmaarrfk]

Try to go even further, there are a few references to OS X

[amorehead]

I think I got most of them, the only ones I can't update are the download URLs.

[hmaarrfk]

I think I got most of them, the only ones I can't update are the download URLs.

Actually reverted the one to the "filename previews". I guess for the things that allude to the download names we can't really change, but for what refers to the operating system in prose we can.

[hmaarrfk]

Thanks!