Docs for Vue3 need to be updated

[shershen08]

code example from main README.md doesn't alow to work in Vue3

in the docs:

<smart-widget-grid :layout="layout">
  <smart-widget slot="0" simple>
    <div class="layout-center">
      <h3>Simple Widget Without Header</h3>
    </div>
  </smart-widget>

While the correct code is in https://github.com/xiaoluoboding/vue-smart-widget/blob/next/app/views/Home.vue

 <smart-widget-grid :layout="layout">
          <template #0>
            <smart-widget simple>
              <div class="layout-center">
                <h3>Simple Widget Without Header</h3>
              </div>
            </smart-widget>
          </template>

see the extra <template #0> tag