Fix instructions for writing the ZFS bootcode on EFI systems

The previous instructions used "gpart bootcode" which do not work
anymore and result in broken boot on EFI systems. Change the
instructions to mount the EFI partition first and then copy the boot
loader file into the target directory on it. Add a note that the target
file is architecture specific, pointing users to uefi(8) where the file
names are listed per architecture.

PR: 293912
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D56643