kexec: update URL of kexec homepage

The referenced html file does not exist anymore. Replace the URL with
the current project homepage.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>