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/[email protected]
Signed-off-by: Chisheng Chen <[email protected]>
Cc: Hsin Chang Yu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: rota1001

lib/rbtree.c

@@ -297,9 +297,9 @@ ____rb_erase_color(struct rb_node *parent, struct rb_root *root,
 				 *   / \           / \
 				 *  N   S    -->  N   sl
 				 *     / \             \
-				 *    sl  sr            S
+				 *    sl  Sr            S
 				 *                       \
-				 *                        sr
+				 *                        Sr
 				 *
 				 * Note: p might be red, and then both
 				 * p and sl are red after rotation(which
@@ -312,9 +312,9 @@ ____rb_erase_color(struct rb_node *parent, struct rb_root *root,
 				 *   / \            /  \
 				 *  N   sl   -->   P    S
 				 *       \        /      \
-				 *        S      N        sr
+				 *        S      N        Sr
 				 *         \
-				 *          sr
+				 *          Sr
 				 */
 				tmp1 = tmp2->rb_right;
 				WRITE_ONCE(sibling->rb_left, tmp1);