[LOUDSTree] Fix type error

src/trees/bp_tree.rs

@@ -481,7 +481,7 @@ mod tests {
     #[test]
     fn from_empty_id_tree() {
         let id_tree: Tree<String> = TreeBuilder::new().with_node_capacity(5).build();
-        let bp_tree: Result<BPTree<i32>, EmptyTreeError> = BPTree::from_id_tree(id_tree);
+        let bp_tree: Result<BPTree<String>, EmptyTreeError> = BPTree::from_id_tree(id_tree);
         assert_eq!(bp_tree.unwrap_err(), EmptyTreeError);
     }