Minor wording suggestion in do_overclock (#91)

Fixes #92

raspi-config

@@ -573,7 +573,7 @@ set_memory_split() {
 
 do_overclock() {
   if ! is_pione && ! is_pitwo; then
-    whiptail --msgbox "This Pi cannot be overclocked." 20 60 2
+    whiptail --msgbox "Only Pi 1 or Pi 2 can be overclocked with this tool." 20 60 2
     return 1
   fi
   if [ "$INTERACTIVE" = True ]; then