docs: add demo for dynamic select options in form essentials (#2618)

Author: jay-es

src/guide/essentials/forms.md

@@ -11,6 +11,12 @@ const checkedNames = ref([])
 const picked = ref('')
 const selected = ref('')
 const multiSelected = ref([])
+const dynamicSelected = ref('A')
+const options = ref([
+  { text: 'One', value: 'A' },
+  { text: 'Two', value: 'B' },
+  { text: 'Three', value: 'C' }
+])
 </script>
 
 # フォーム入力バインディング {#form-input-bindings}
@@ -340,23 +346,32 @@ export default {
 </div>
 
 ```vue-html
+<div>Selected: {{ selected }}</div>
+
 <select v-model="selected">
   <option v-for="option in options" :value="option.value">
     {{ option.text }}
   </option>
 </select>
-
-<div>Selected: {{ selected }}</div>
 ```
+<div class="demo">
+  <div>Selected: {{ dynamicSelected }}</div>
+  
+  <select v-model="dynamicSelected">
+    <option v-for="option in options" :value="option.value">
+      {{ option.text }}
+    </option>
+  </select>
+</div>
 
 <div class="composition-api">
 
-[Playground で試す](https://play.vuejs.org/#eNplkMFugzAQRH9l5YtbKYU7IpFoP6CH9lb3EMGiWgLbMguthPzvXduEJMqNYUazb7yKxrlimVFUop5arx3BhDS7kzJ6dNYTrOCxhwC9tyNIjkpllGmtmWJ0wJawg2MMPclGPl9N60jzx+Z9KQPcRfhHFch3g/IAy3mYkVUjIRzu/M9fe+O/Pvo/Hm8b3jihzDdfr8s8gwewIBzdcCZkBVBnXFheRtvhcFTiwq9ECnAkQ3Okt54Dm9TmskYJqNLR3SyS3BsYct3CRYSFwGCpusx/M0qZTydKRXWnl9PHBlPFhv1lQ6jL6MZl+xoR/gFjPZTD)
+[Playground で試す](https://play.vuejs.org/#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)
 
 </div>
 <div class="options-api">
 
-[Playground で試す](https://play.vuejs.org/#eNp1kMFqxCAQhl9l8JIWtsk92IVtH6CH9lZ7COssDbgqZpJdCHn3nWiUXBZE/Mdvxv93Fifv62lE0Qo5nEPv6ags3r0LBBov3WgIZmUBdEfdy2s6AwSkMdisAAY0eCbULVSn6pCrzlPv7NDCb64AzEB4J+a+LFYHmDozYuyCpfTtqJ+b21Efz6j/gPtpn8xl7C8douaNl2xKUhaEV286QlYAMgWB6e3qNJp3JXIyJSLASErFyMUFBjbZ2xxXCWijkXJZR1kmsPF5g+s1ACybWdmkarLSpKejS0VS99Pxu3wzT8jOuF026+2arKQRywOBGJfE)
+[Playground で試す](https://play.vuejs.org/#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)
 
 </div>