Documentation/00-INDEX: remove reference to exception.txt

The exception.txt has been removed from the Documentation directory.  So
update the index file for it.

Signed-off-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>