update brew mac code example

Example 1: update homebrew

brew update

Example 2: install brew on mac

/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

Example 3: install brew on mac

export HOMEBREW_BREW_GIT_REMOTE="..."  # put your Git mirror of Homebrew/brew here
export HOMEBREW_CORE_GIT_REMOTE="..."  # put your Git mirror of Homebrew/homebrew-core here
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"