Clarify homebrew installation instructions

[felliott]

This PR combines two small doc patches. The first removes a duplicate paragraph from the homebrew setup instructions. The second tells users who already have homebrew installed to chown /usr/local and /opt to themselves to avoid permissions issues. It's written for single-user system, but I can add more text clarifying in which situations this shouldn't be done e.g. don't do it if you already own it, don't do it on a shared server, etc.

Cheers, Fitz

[sloria]

Sorry for the delay in responding to this.

Just one change to make, and this is good to merge.

[sloria]

Thanks!