Change waybar clock format

Following an update to waybar, the clock starting displaying fractional
seconds. Changing seconds from `%S` to `%OS` truncates the fractional
second, though this is possibly not portable as `%OS` is "alternative
second display" and the definition of "alternative" is vague.

.config/waybar/config

@@ -20,7 +20,7 @@
 	},
 	"clock": {
 		"interval": 5,
-		"format": "{:%a %b %d %H:%M:%S}",
+		"format": "{:%a %b %d %R:%OS}",
 		"tooltip-format": "<big>{:%Y %B}</big>\n<tt><small>{calendar}</small></tt>",
 		"format-alt": "{:%Y-%m-%d}"
 	},