fix: introduce the wrapper object to the props default example (#492)

Author: skirtles-code

src/guide/migration/props-default-this.md

@@ -19,11 +19,13 @@ import { inject } from 'vue'
 
 export default {
   props: {
-    theme: props => {
-      // `props` is the raw values passed to the component,
-      // before any type / default coercions
-      // can also use `inject` to access injected properties
-      return inject('theme', 'default-theme')
+    theme: {
+      default (props) {
+        // `props` is the raw values passed to the component,
+        // before any type / default coercions
+        // can also use `inject` to access injected properties
+        return inject('theme', 'default-theme')
+      }
     }
   }
 }