arm64: Update README

Drop sample command line to use against the ARM Foundation v8 model and
replace the path to something that matches LEDE's directory structure.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

target/linux/arm64/README

-This multi-platform ARMv8 target can be used either with ARM Ltd.'s Foundation_V8
-fast-model doing the following:
-
-Foundation_v8 --image bin/arm64-eglibc/openwrt-arm64-vexpress-foundation.axf
-
-or you can also use QEMU:
+This multi-platform ARMv8 target can be used with QEMU:
 
 qemu-system-aarch64 -machine virt -cpu cortex-a57 -machine type=virt -nographic \
 	-smp 1 -m 2048 \
-	-kernel bin/arm64-eglibc/openwrt-arm64-qemu-virt-initramfs.Image \
+	-kernel bin/targets/arm64/generic/lede-arm64-qemu-virt-initramfs.Image \
 	--append "console=ttyAMA0"
 
 and enjoy the system booting.