docs: 📝 fix dead link in theme.md

Author: luthfimasruri

docs/content/guide/themes.md

@@ -14,7 +14,7 @@ import '@vueup/vue-quill/dist/vue-quill.bubble.css';
 
 These stylesheets can be found in the Quill distribution, but for convenience, they are also linked in VueQuill's `dist` folder.
 
-Then, pass the name of the theme to the [`theme prop`](../api/props.md).
+Then, pass the name of the theme to the [`theme prop`](../api/index.md).
 
 ~~~ vue
 <template>