message: docs: fix custom handler header
diff: @@ -268,7 +268,7 @@ class Error extends React.Component {
  export default Error;
  ```
  -### Custom lambda handler
  +## Custom lambda handler
  If you need to customize the lambda handler you can do so by providing a path to your own handler in the `customHandler` field. Note that it resolves the path to the custom handler relative to your `next.config.js`.
  