Bump copyright year of the Developers' Handbook due to several content
changes back in 2007 (e.g. [1], [2]) and 2008 (e.g. [3], [4], [5])

[1] http://www.freebsd.org/cgi/cvsweb.cgi/doc/en_US.ISO8859-1/books/developers-handbook/ipv6/chapter.sgml?rev=1.19;content-type=text%2Fx-cvsweb-markup
[2] http://www.freebsd.org/cgi/cvsweb.cgi/doc/en_US.ISO8859-1/books/developers-handbook/kerneldebug/chapter.sgml?rev=1.75;content-type=text%2Fx-cvsweb-markup
[3] http://www.freebsd.org/cgi/cvsweb.cgi/doc/en_US.ISO8859-1/books/developers-handbook/secure/chapter.sgml?rev=1.29;content-type=text%2Fx-cvsweb-markup
[4] http://www.freebsd.org/cgi/cvsweb.cgi/doc/en_US.ISO8859-1/books/developers-handbook/book.sgml?rev=1.56;content-type=text%2Fx-cvsweb-markup
[5] http://www.freebsd.org/cgi/cvsweb.cgi/doc/en_US.ISO8859-1/books/developers-handbook/policies/chapter.sgml?rev=1.35;content-type=text%2Fx-cvsweb-markup