Update guestbook.md

The previous gcutil command fails to connect:

lynchb-macbookpro2:kubernetes lynchb$ gcutil ssh kubernetes-minion-3 --zone us-central1-b
INFO: Zone for kubernetes-minion-3 detected as us-central1-b.
INFO: Running command line: ssh -o UserKnownHostsFile=/dev/null -o CheckHostIP=no -o StrictHostKeyChecking=no -i /Users/lynchb/.ssh/google_compute_engine -A -p 22 lynchb@23.236.61.38 -- --zone us-central1-b
Warning: Permanently added '23.236.61.38' (RSA) to the list of known hosts.
bash: --: invalid option
Usage:	bash [GNU long option] [option] ...
	bash [GNU long option] [option] script-file ...
GNU long options:
	--debug
	--debugger
	--dump-po-strings
	--dump-strings
	--help
	--init-file
	--login
	--noediting
	--noprofile
	--norc
	--posix
	--protected
	--rcfile
	--restricted
	--verbose
	--version
Shell options:
	-irsD or -c command or -O shopt_option		(invocation only)
	-abefhkmnptuvxBCHP or -o option
you could remove the zone, gcutil can guess it.