Fix null check

Check the value, not its name (which is never null). Found by new CA2264 analyser in .NET 9 SDK.

PollyDemos/Demo11_MultipleConcurrencyLimiters.cs

@@ -61,7 +61,7 @@ public class Demo11_MultipleConcurrencyLimiters : ConcurrencyLimiterDemoBase
 
     public override async Task ExecuteAsync(CancellationToken externalCancellationToken, IProgress<DemoProgress> progress)
     {
-        ArgumentNullException.ThrowIfNull(nameof(progress));
+        ArgumentNullException.ThrowIfNull(progress);
 
         PrintHeader(progress);
         TotalRequests = 0;