Update services.md -- wrong method name in example code (#20204)

aspnetcore/grpc/services.md

@@ -71,7 +71,7 @@ By default the generated `GreeterBase` doesn't do anything. Its virtual `SayHell
 ```csharp
 public class GreeterService : GreeterBase
 {
-    public override Task<HelloReply> UnaryCall(HelloRequest request, ServerCallContext context)
+    public override Task<HelloReply> SayHello(HelloRequest request, ServerCallContext context)
     {
         return Task.FromResult(new HelloRequest { Message = $"Hello {request.Name}" });
     }