Remove what you put into the file and try to start rtorrent again with :
systemctl restart rtorrent@YourUser.service (in case of, the .rtorrent.rc template file : here)
If that works, make a copy of your .rtorrent.rc :
cp .rtorrent.rc .rtorrent.rc.bkp
After that, you can modify your .rtorrent.rc file to make some modifications.
If it fails to start rtorrent, simply do :
cp .rtorrent.rc.bkp .rtorrent.rc
And restart rtorrent for your user again