Make CCore benchmark server write consistent startup message (#729)

Author: JamesNK

perf/benchmarkapps/GrpcCoreServer/Program.cs

@@ -73,7 +73,7 @@ private static async Task RunAsync(IConfigurationRoot config)
             };
             server.Start();
 
-            Console.WriteLine("Started!");
+            Console.WriteLine("Application started.");
             Console.WriteLine("Press any key to stop the server...");
             Console.ReadKey();