Predefine do_insert instead of do_delete in erlang_extraction

Because writing the insert function is the second task,
I think it's more beneficial as a hint to predefine the private do_ function there
so that beginners are not stuck on the different order of arguments.
And then we can let the student figure this out on their own for the delete function

exercises/concept/erlang-extraction/src/erlang_extraction.gleam

@@ -8,10 +8,10 @@ pub fn insert(tree: GbTree(k, v), key: k, value: v) -> GbTree(k, v) {
   todo
 }
 
-pub fn delete(tree: GbTree(k, v), key: k) -> GbTree(k, v) {
+fn do_insert(key: k, value: v, tree: GbTree(k, v)) -> GbTree(k, v) {
   todo
 }
 
-fn do_delete(key: k, tree: GbTree(k, v)) -> GbTree(k, v) {
+pub fn delete(tree: GbTree(k, v), key: k) -> GbTree(k, v) {
   todo
 }