include: drivers: led: Correct comment for struct led_info

The comments for `index` and `num_colors` in `struct led_info` were
mistakenly swapped. This commit corrects them to accurately reflect their
actual purpose.

Signed-off-by: Chen Xingyu <hi@xingrz.me>

include/zephyr/drivers/led.h

@@ -36,9 +36,9 @@ extern "C" {
 struct led_info {
 	/** LED label */
 	const char *label;
-	/** Number of colors per LED */
-	uint32_t index;
 	/** Index of the LED on the controller */
+	uint32_t index;
+	/** Number of colors per LED */
 	uint8_t num_colors;
 	/** Mapping of the LED colors */
 	const uint8_t *color_mapping;