Replace dpms commands with power in sample command

sway-output(5) lists `dpms` as deprecated and an alias for `power`. For
clarity and consistency, the `swayidle(1)` man page should use `power`
in the example command

Author: laurelmay

swayidle.1.scd

@@ -86,8 +86,8 @@ swayidle responds to the following signals:
 ```
 swayidle -w \\
 	timeout 300 'swaylock -f -c 000000' \\
-	timeout 600 'swaymsg "output * dpms off"' \\
-		resume 'swaymsg "output * dpms on"' \\
+	timeout 600 'swaymsg "output * power off"' \\
+		resume 'swaymsg "output * power on"' \\
 	before-sleep 'swaylock -f -c 000000'
 ```