Update mikrotik-routeros.sh (#3629)

Update the ZIP extracted from 7.15.2 to 7.15.3, keeping it up to date in the stable branch of the routerOS

vm/mikrotik-routeros.sh

@@ -234,7 +234,7 @@ msg_ok "Using ${CL}${BL}$STORAGE${CL} ${GN}for Storage Location."
 msg_ok "Virtual Machine ID is ${CL}${BL}$VMID${CL}."
 msg_info "Getting URL for Mikrotik RouterOS CHR Disk Image"
 
-URL=https://download.mikrotik.com/routeros/7.15.2/chr-7.15.2.img.zip
+URL=https://download.mikrotik.com/routeros/7.15.3/chr-7.15.3.img.zip
 
 sleep 2
 msg_ok "${CL}${BL}${URL}${CL}"