message: docs: fix a link to plugin-packer in README
diff: @kintone/plugin-manifest-validator
  ====
  -Validate `manifest.json` of kintone plugin. Used in [kintone-plugin-packer](https://github.com/teppeis/kintone-plugin-packer).
  +Validate `manifest.json` of kintone plugin. Used in [@kintone/plugin-packer](https://github.com/kintone/plugin-packer).
  [![npm version][npm-image]][npm-url]
  ![Node.js Version Support][node-version]
  