Change the 'idKey' parameter's default value (#30502)

I think it is easier for beginners to use the function, because the default value for the id key is 'id', not 'key'

packages/gatsby-source-wordpress/docs/tutorials/using-wordpress-menus.md

@@ -163,7 +163,7 @@ To do this, we can use a function like the following:
 ```javascript
 const flatListToHierarchical = (
   data = [],
-  { idKey = "key", parentKey = "parentId", childrenKey = "children" } = {}
+  { idKey = "id", parentKey = "parentId", childrenKey = "children" } = {}
 ) => {
   const tree = []
   const childrenOf = {}