Fix the Zipkin exporter registration (#2135)

There was a missing line to set the service name similar to other places

examples/AspNetCore/Startup.cs

@@ -70,6 +70,7 @@ namespace Examples.AspNetCore
                     break;
                 case "zipkin":
                     services.AddOpenTelemetryTracing((builder) => builder
+                        .SetResourceBuilder(ResourceBuilder.CreateDefault().AddService(this.Configuration.GetValue<string>("Zipkin:ServiceName")))
                         .AddAspNetCoreInstrumentation()
                         .AddHttpClientInstrumentation()
                         .AddZipkinExporter());