docs: Add a missing type symbol to the key

[Yukon123]

Description of Problem

export type VNodeProps = {
  key?: string | number | symbol
  ref?: VNodeRef

  // vnode hooks
  onVnodeBeforeMount?: VNodeMountHook | VNodeMountHook[]
  onVnodeMounted?: VNodeMountHook | VNodeMountHook[]
  onVnodeBeforeUpdate?: VNodeUpdateHook | VNodeUpdateHook[]
  onVnodeUpdated?: VNodeUpdateHook | VNodeUpdateHook[]
  onVnodeBeforeUnmount?: VNodeMountHook | VNodeMountHook[]
  onVnodeUnmounted?: VNodeMountHook | VNodeMountHook[]
}

https://github.com/vuejs/vue-next/blob/v3.2.11/packages/runtime-core/src/vnode.ts#L92

Proposed Solution

Additional Information