docs: adjust plugin dev guide example to write modified contents, fixes #3655

[akrawchyk]

fixes #3655

[NataliaTepluhina]

@akrawchyk thank you so much for spotting this! I think I missed

 contentMain = lines.join('\n');

in my example but including end of line makes it even better.

[akrawchyk]

@NataliaTepluhina no prob 😄 , I can update the example to use either EOL or \n everywhere, up to you.

[NataliaTepluhina]

I think EOL is more consistent in this case so let's leave it your way 😉

@sodatea please have a final look 🙇‍♀️

[haoqunjiang]

The best option is to maintain the line-ending (like prettier prettier/prettier#472)
But that may be too much for an example snippet so I think EOL is good enough.

(sidenote: Airbnb Style Guide enforces LF as line ending, while create-react-app uses os.EOL everywhere facebook/create-react-app#6610, other common style guides has no strong opinion on this, which means most people may just use os default)