change thoughtbot/rcm to thoughtbot/formulae

Started reading the thoughtbot playbook last week, which led me to laptop. Calling mac from Terminal went fine untill the last installation which yielded "ERROR: no available formula for rcm". Googling the error message brought me to Matthew Sumner's commit # 2915cf1f696777a7438d0bbf92f688f32e1e8264 in the dotfile repo. Applying the same modif to the laptop mac shell script got rid of the error message.