docs: make v-model code example more readable (#3069)

Author: FloEdelmann

src/guide/components/v-model.md

@@ -99,11 +99,15 @@ const model = defineModel({ default: 0 })
 :::warning
 If you have a `default` value for `defineModel` prop and you don't provide any value for this prop from the parent component, it can cause a de-synchronization between parent and child components. In the example below, the parent's `myRef` is undefined, but the child's `model` is 1:
 
+**Child component:**
+
 ```js
-// child component:
 const model = defineModel({ default: 1 })
+```
 
-// parent component:
+**Parent component:**
+
+```js
 const myRef = ref()
 ```