Add required sh(1) to synopsis

Add required sh(1) to synopsis

Contrary to other scripts in base like rc.d(8) or MAKEDEV(8), netstart(8)
itself is not executable and must be passed as file to sh(1):
	$ man -h netstart
	/etc/netstart [-n] [interface ...]
	$ /etc/netstart
	ksh: /etc/netstart: cannot execute - Permission denied

Fix usage and synopsis to provide required usage:
	$ man -h netsart
	sh /etc/netstart [-n] [interface ...]

OK jmc