Update components-props.md (#2728)

Author: JasperWoo

src/v2/guide/components-props.md

@@ -347,6 +347,7 @@ This pattern allows you to use base components more like raw HTML elements, with
 
 ```html
 <base-input
+  label="Username:"
   v-model="username"
   required
   placeholder="Enter your username"