linux: Fix syscall list generation instructions

Make the instructions for syscall list generation match Makefile and
refer to `update-syscall-lists'; there has been no `update-arch-syscall'
target.  Also use single quotes around the command to stick to the ASCII
character set.

Fixes 4cf0d223 ("Linux: Add tables with system call numbers").

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>