lib/rbtree.c: fix typo in comment of rb_insert_augmented()

The function name in the comment is not correct.

Link: http://lkml.kernel.org/r/20181010021344.60433-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>