adjust plugin dev guide example to write modified contents; fixes(#3655)

docs/dev-guide/plugin-dev.md

@@ -234,18 +234,19 @@ api.onCreateComplete(() => {
 
 Finally, you need to write the content back to the main file:
 
-```js{11}
+```js{2,11}
 // generator/index.js
 
 api.onCreateComplete(() => {
+ const { EOL } = require('os')
  const fs = require('fs')
  const contentMain = fs.readFileSync(api.entryFile, { encoding: 'utf-8' })
  const lines = contentMain.split(/\r?\n/g)
 
  const renderIndex = lines.findIndex(line => line.match(/render/))
  lines[renderIndex] += `\n router,`
 
- fs.writeFileSync(api.entryFile, contentMain, { encoding: 'utf-8' })
+ fs.writeFileSync(api.entryFile, lines.join(EOL), { encoding: 'utf-8' })
 })
 ```