use --:-- for loading timestamps instead of 00:00 (#37607)

"00:00" looks like a zero length replay, when really it's still loading. 
I like "--:--" for loading instead, it feels more like a placeholder instead of a zero-value.

static/app/components/replays/replayController.tsx

@@ -114,7 +114,7 @@ function ReplayCurrentTime() {
 
   return (
     <span>
-      {formatTime(currentTime)} / {durationMs ? formatTime(durationMs) : '??:??'}
+      {formatTime(currentTime)} / {durationMs ? formatTime(durationMs) : '--:--'}
     </span>
   );
 }