external/common: Fix grammar in arch_flash error message
external/common: Fix grammar in arch_flash error message

Signed-off-by: Samuel Mendoza-Jonas <ce35fd21fe38dbd75c01124bec431654a5f8eea9@au1.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

stewart-ibm/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,legoater/skiboot,legoater/skiboot,shenki/skiboot,mikey/skiboot,legoater/skiboot,legoater/skiboot,mikey/skiboot,open-power/skiboot,stewart-ibm/skiboot,qemu/skiboot,qemu/skiboot,open-power/skiboot,open-power/skiboot,stewart-ibm/skiboot,qemu/skiboot,shenki/skiboot,mikey/skiboot,open-power/skiboot