lib/rbtree.c: fix the example typo

Replace the "Sr" with "sr", the example is wrong if sl and N don't have
child nodes, so sr should be red node.

Link: https://lkml.kernel.org/r/20240628142229.69419-1-zxcvb600870024@gmail.com


Signed-off-by: Hsin Chang Yu <zxcvb600870024@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>