fix keep-alive examples (vuejs#1444)

Author: skirtles-code

src/guide/built-ins/keep-alive-demos/SwitchComponent.vue

@@ -4,20 +4,20 @@ import CompB from './CompB.vue'
 
 let current = $shallowRef(CompA)
 
-const { useKeepAlive } = defineProps(['useKeepAlive'])
+const { useKeepAlive } = defineProps({ useKeepAlive: Boolean })
 </script>
 
 <template>
   <div class="demo">
     <label><input type="radio" v-model="current" :value="CompA" /> A</label>
     <label><input type="radio" v-model="current" :value="CompB" /> B</label>
     <template v-if="useKeepAlive">
-      <component :is="current"></component>
-    </template>
-    <template v-else>
       <KeepAlive>
         <component :is="current"></component>
       </KeepAlive>
     </template>
+    <template v-else>
+      <component :is="current"></component>
+    </template>
   </div>
 </template>