Update contributing article to newer link

[Eitan Adler]

I rewrote the "ideas page" on the wiki per a recent thread
(http://www.mail-archive.com/freebsd-hackers@freebsd.org/msg70181.html).
On this page: http://www.freebsd.org/doc/en/articles/contributing/article.html
the link in section 1.4 is currently pointing to
http://www.freebsd.org/projects/ideas/ and the new page is
http://wiki.freebsd.org/IdeasPage
I also think that http://www.freebsd.org/projects/ideas/ should
redirect to the new page.