lib/radix-tree.c: kernel-doc warning fix

index has been removed from __radix_tree_delete_node in 449dd698
("mm: keep page cache radix tree nodes in check")

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>