docs: fix example changing main file

[beliolfa]

In the giving example

  fs.writeFileSync(api.entryFile, contentMain, { encoding: 'utf-8' })

it was being written back the original content instead of the modified lines. I've changed the approach adding the new line to the array and then joining it back.

  lines.splice(renderIndex, 0, `  router,`)
  const newContent = lines.join('\r\n')
  fs.writeFileSync(api.entryFile, newContent, { encoding: 'utf-8' })

[haoqunjiang]

Thanks for this pull request and sorry for the late reply.
But we found #3656 to be a better fix for this example.
Thanks again though, your help is much appreciated.