Highlighting change of the default config key.

While migrating I failed to spot this change and almost caused a prod bug, so I'm proposing this change to avoid anyone else also doing so 👍

Author: jonathonhawkins92

docs/src/pages/guides/migrating-to-react-query-3.md

@@ -48,6 +48,8 @@ const queryClient = new QueryClient()
 
 Default options for queries and mutations can now be specified in `QueryClient`:
 
+**Notice that it's now defaultOptions instead of defaultConfig**
+
 ```js
 const queryClient = new QueryClient({
   defaultOptions: {