docs: fix code highlighting in preload tutorial
The highlighted lines in the code snippets were unaligned,
which could cause a newcomer unneeded confusion on what
lines need to be changed.
Fix incorrect highlight in an example snippet
At the moment, the "Communicating between processes" `main.js` snippet highlights the line containing `})` when the relevant line is `ipcMain.handle('ping', () => 'pong')`.
