[docs] Example for "Events" in Guides has inconsistent payload from API doc

[QinBa]

In the example at https://tauri.studio/en/docs/guides/events/, emit takes javascript object as payload, see

emit('click', {
  theMessage: 'Tauri is awesome!'
})

However, in the actual API session on "event", see https://tauri.studio/en/docs/api/js/modules/event#emit. It says the payload should be "string".

I tried it out in an minimal example, it seems to me that I have to use string rather than js object.