[PATCH] DocBook: fix html link

The start page for each book has changed from book1.html to index.html.
Update our generated links acocrdingly.

Signed-off-by: Martin Waitz <tali@admingilde.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>