Fix PDF export

Fix the bug puppeteer to export the guide to PDF (plugin: https://github.com/ulivz/vuepress-plugin-export).

Error:

```
info Start to generate current site to PDF ...
Error: Failed to launch chrome! spawn
/dist/node_modules/puppeteer/.local-chromium/linux-686378/chrome-linux/chrome
ENOENT
TROUBLESHOOTING: https://github.com/GoogleChrome/puppeteer/blob/master/docs/troubleshooting.md
```