AP_Arming: rename contains_terrain_relative->contains_terrain_alt_items

can we rename "constains_terrain_relative" to "contains_terrain_alt"? No big deal of course but "terrain_alt" is what we use in Copter so it's more likely to show up in searches.

libraries/AP_Arming/AP_Arming.cpp

@@ -1027,7 +1027,7 @@ bool AP_Arming::terrain_database_required() const
         // no mission support?
         return false;
     }
-    if (mission->contains_terrain_relative()) {
+    if (mission->contains_terrain_alt_items()) {
         return true;
     }
     return false;