That error is not very informative, eh?
Without anything to go on, work around the editor syntax checking by downloading the file to your local computer via FTP or your hosting file manager. Keep a backup copy of the original. Make the change using a plain text editor, then upload back.
If your site has trouble with the edit, either check your error log for a specific error message, or define WP_DEBUG in wp-config.php as true, then reload your site to see the error message in your browser. Correct the described error or restore the file from your backup copy.
