docs: fix lastName type in cookbook

Closes #428

Author: kevinmarrec

docs/cookbook/components/script.class-api.ts

@@ -2,7 +2,7 @@ import { Vue, Component, Prop } from 'vue-property-decorator'
 
 interface User {
   firstName: string
-  lastName: number
+  lastName: string
 }
 
 @Component

docs/cookbook/components/script.composition-api.ts

@@ -2,7 +2,7 @@ import { defineComponent, computed, ref } from '@vue/composition-api'
 
 interface User {
   firstName: string
-  lastName: number
+  lastName: string
 }
 
 export default defineComponent({

docs/cookbook/components/script.options-api.ts

@@ -2,7 +2,7 @@ import Vue, { PropOptions } from 'vue'
 
 interface User {
   firstName: string
-  lastName: number
+  lastName: string
 }
 
 export default Vue.extend({