Fix gRPC typo (dotnet#16984)

Author: JamesNK

aspnetcore/grpc/index.md

@@ -105,7 +105,7 @@ gRPC clients are concrete client types that are [generated from *\*.proto* files
 var channel = GrpcChannel.ForAddress("https://localhost:5001");
 var client = new Greeter.GreeterClient(channel);
 
-var response = await client.SayHello(
+var response = await client.SayHelloAsync(
     new HelloRequest { Name = "World" });
 
 Console.WriteLine(response.Message);