Updated homebrew install instructions

As of https://github.com/Homebrew/homebrew/commit/5efa7a6be49015e3b327da0a215b9b5d827a69fb and https://github.com/Homebrew/homebrew/commit/16dd36bffc5d8ab0b1b8fd8cd240097bca5fcf2e there is a homebrew formula for libswift (https://github.com/Homebrew/homebrew/pull/31420). Therefore the install documentation should be updated.