docs: fix missing slash (#1306)

Author: itshizhan

docs/content/3.guide/1.writing/3.mdc.md

@@ -60,7 +60,7 @@ A component's slots can accept content or another components.
   <template>
     <section>
       <h1 class="text-4xl"><slot /></h1>
-      <slot name="description">
+      <slot name="description"/>
     </section>
   </template>
   ```
@@ -183,9 +183,15 @@ The `{}` identifier passes props to components in a terse way by using a `key=va
   ```
 
   ```html [Alert.vue]
+<script setup>
+defineProps(['type'])
+</script>
+
+<template>
   <div :class="[type]">
     <Markdown unwrap="p" />
   </div>
+</template>
   ```
 
   ::code-block{label="Preview" preview}