Update install.rst

[andreamorales]

Changed Mac installation instructions after going through them with @pshriwise and finding Source is the only way to install for Mac (at least on my mac M1).

There might be some extra line breaks due to trying to mirror your formatting on the text file. Let me know if I should get rid of those line breaks and I'm happy to modify this PR.

Description

Please include a summary of the change and which issue is fixed if applicable. Please also include relevant motivation and context.

Fixes # (issue)

Checklist

• [ ] I have performed a self-review of my own code
• [ ] I have run clang-format (version 15) on any C++ source files (if applicable)
• [ ] I have followed the style guidelines for Python source files (if applicable)
• [ ] I have made corresponding changes to the documentation (if applicable)
• [ ] I have added tests that prove my fix is effective or that my feature works (if applicable)