fix: missing package.json

Author: CoderOMaster

vitepress-plugin-moss/demo-site/package.json

@@ -8,7 +8,7 @@
   },
   "scripts": {
     "docs:dev": "vitepress dev docs",
-    "predocs:build": "pnpm -C .. run build",
+    "predocs:build": "pnpm -C .. install && pnpm -C .. run build",
     "docs:build": "vitepress build docs",
     "docs:preview": "vitepress preview docs"
   },