[docs] add !kPSK to the cipher string recommendation as the comments before already mention

From: Stefan Bühler <stbuehler@web.de>

git-svn-id: svn://svn.lighttpd.net/lighttpd/branches/lighttpd-1.4.x@2933 152afb58-edef-0310-8abb-c4023f1b3aa9