When setting up bitcoin core on my new MacBook via the macOS Build Instructions and Notes, running touch "/Users/${USER}/Library/Application Support/Bitcoin/bitcoin.conf" resulted in No such file or directory because my /Users/${USER}/Library/Application Support/Bitcoin/ folder had not been created yet.
This PR adds mkdir "/Users/${USER}/Library/Application Support/Bitcoin" to the documentation before creating the configuration file with touch "/Users/${USER}/Library/Application Support/Bitcoin/bitcoin.conf"