Update forms.md (#2420)

Added a key property to line 344 on the select option dynamic rendering.

Author: Sam Vitare

src/guide/essentials/forms.md

@@ -341,7 +341,7 @@ export default {
 
 ```vue-html
 <select v-model="selected">
-  <option v-for="option in options" :value="option.value">
+  <option v-for="option in options" :key="option.value" :value="option.value">
     {{ option.text }}
   </option>
 </select>