rpc: fix typo example code (#20284)

rpc/doc.go

@@ -73,7 +73,7 @@ An example server which uses the JSON codec:
 
  calculator := new(CalculatorService)
  server := NewServer()
- server.RegisterName("calculator", calculator")
+ server.RegisterName("calculator", calculator)
 
  l, _ := net.ListenUnix("unix", &net.UnixAddr{Net: "unix", Name: "/tmp/calculator.sock"})
  for {