lib/rbtree.c: fix the example typo

Replace `sr` with `Sr`.  The condition `!tmp1 || rb_is_black(tmp1)`
ensures that `tmp1` (which is `sibling->rb_right`) is either NULL or a
black node.  Therefore, the right child of the sibling must be black, and
the example should use `Sr` instead of `sr`.

Link: https://lkml.kernel.org/r/20250403112614.570140-1-johnny1001s000602@gmail.com


Signed-off-by: Chisheng Chen <johnny1001s000602@gmail.com>
Cc: Hsin Chang Yu <zxcvb600870024@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>