revert one more RateLimiterHelper.GetElapsedTime

Author: WeihanLi

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/FixedWindowRateLimiter.cs

@@ -39,7 +39,7 @@ public sealed class FixedWindowRateLimiter : ReplenishingRateLimiter
         private static readonly RateLimitLease FailedLease = new FixedWindowLease(false, null);
 
         /// <inheritdoc />
-        public override TimeSpan? IdleDuration => Stopwatch.GetElapsedTime(_idleSince);
+        public override TimeSpan? IdleDuration => RateLimiterHelper.GetElapsedTime(_idleSince);
 
         /// <inheritdoc />
         public override bool IsAutoReplenishing => _options.AutoReplenishment;