doc: fix composition-api-setup code indent (#496)

Author: xinpingwang

src/guide/composition-api-setup.md

@@ -42,9 +42,9 @@ export default {
 import { toRefs } from 'vue'
 
 setup(props) {
-	const { title } = toRefs(props)
+  const { title } = toRefs(props)
 
-	console.log(title.value)
+  console.log(title.value)
 }
 ```
 
@@ -55,8 +55,8 @@ setup(props) {
 // MyBook.vue
 import { toRef } from 'vue'
 setup(props) {
-	const title = toRef(props, 'title')
-	console.log(title.value)
+  const title = toRef(props, 'title')
+  console.log(title.value)
 }
 ```