Update show_marks documentation

sway/sway.5.scd

@@ -451,10 +451,10 @@ You can get a list of output names with *swaymsg -t get\_outputs*. You may also
 match any output by using the output name "\*". Be sure to add this output
 config after the others, or it will be matched instead of the others.
 
-*show\_marks* on|off
-	If *show\_marks* is on, marks will be displayed in the window borders.
-	Any mark that starts with an underscore will not be drawn even if the
-	option is on. The default is _on_.
+*show\_marks* yes|no
+	If *show\_marks* is yes, marks will be displayed in the window borders.
+	Any mark that starts with an underscore will not be drawn even if
+	*show\_marks* is yes. The default is _yes_.
 
 *opacity* <value>
 	Set the opacity of the window between 0 (completely transparent) and 1