bitbake: bitbake-user-manual/bitbake-user-manual-intro.xml: Fixed link

bitbake: bitbake-user-manual/bitbake-user-manual-intro.xml: Fixed link

I inadvertantly used a &YOCTO_* variable in the book to get
a link to the landing YP page.  Of course this does not work.
I replaced it with the actual URL.  Unfortunately, a previous
commit slipped through that would have prevented the manual
from "making" correctly.

(Bitbake rev: 63f5a4b06a575e1fb18805b45a01549bffdc4a03)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>