fix(cpu): change main path in package.json

Author: yueshuangyan

packages/paddlejs-backend-cpu/package.json

@@ -2,7 +2,7 @@
   "name": "@paddlejs/paddlejs-backend-cpu",
   "version": "0.0.2-beta.0",
   "description": "",
-  "main": "index.js",
+  "main": "lib/index",
   "scripts": {
     "start": "webpack serve --mode development",
     "build": "webpack --config webpack.prod.js"