Don't use magic constants

diff --git a/TomTom.lua b/TomTom.lua
index 0c9dc01..9948aee 100755
--- a/TomTom.lua
+++ b/TomTom.lua
@@ -474,7 +474,7 @@ end
 -------------------------------------------------------------------]]--

 StaticPopupDialogs["TOMTOM_REMOVE_ALL_CONFIRM"] = {
-	preferredIndex = 3,
+	preferredIndex = STATICPOPUPS_NUMDIALOGS,
     text = L["Are you sure you would like to remove ALL TomTom waypoints?"],
     button1 = L["Yes"],
     button2 = L["No"],