message: docs: improve scroll-behavior.md [skip ci]
  fixing the syntax in scroll behavior function, the `el: '#main'` should be in a comment and the element **el** should be in the return object
diff: @@ -35,10 +35,10 @@ You can also pass a CSS selector or a DOM element via `el`. In that scenario, `t
  const router = createRouter({
  scrollBehavior (to, from, savedPosition) {
  // always scroll 10px above the element #main
  - el: '#main',
  + // el: '#main'
  // could also be
  // el: document.getElementById('main'),
  - return { top: -10 }
  + return { el: '#main', top: -10 }
  }
  })
  ```
  