docs: update query-key docs

add another comment to the defaultQueryFn example

Author: TkDodo

docs/src/pages/guides/default-query-function.md

@@ -7,6 +7,7 @@ If you find yourself wishing for whatever reason that you could just share the s
 
 ```js
 // Define a default query function that will receive the query key
+// the queryKey is guaranteed to be an Array here
 const defaultQueryFn = async ({ queryKey }) => {
   const { data } = await axios.get(`https://jsonplaceholder.typicode.com${queryKey[0]}`);
   return data;