fix: use useData() instead of $site (#365)

Author: CHOYSEN

src/client/theme-default/NotFound.vue

@@ -2,11 +2,14 @@
   <div class="theme">
     <h1>404</h1>
     <blockquote>{{ getMsg() }}</blockquote>
-    <a :href="$site.base" aria-label="go to home">Take me home.</a>
+    <a :href="site.base" aria-label="go to home">Take me home.</a>
   </div>
 </template>
 
 <script setup lang="ts">
+import { useData } from 'vitepress'
+
+const { site } = useData()
 const msgs = [
   `There's nothing here.`,
   `How did we get here?`,