[Doc] Tweak stdio transport example code in the README

The example code is located at `./examples/stdio_server.rb`.
Also, the content provided as input via stdio has been updated to avoid returning an error data.
Using ping pong as an example, here's the difference:

## Before

An error data is returned.

```console
$ ./examples/stdio_server.rb
{"jsonrpc":"2.0","id":"1","result":"pong"}
{"jsonrpc":"2.0","id":null,"error":{"code":-32600,"message":"Invalid Request","data":"Method name must be a string and not start with \"rpc.\""}}
```

This error message is generated in the following location:
https://github.com/Shopify/json-rpc-handler/blob/v0.1.1/lib/json_rpc_handler.rb#L70

## After

The expected result is returned without error data.

```console
$ ./examples/stdio_server.rb
{"jsonrpc":"2.0","id":"1","method":"ping"}
{"jsonrpc":"2.0","id":"1","result":{}}
```

The other stdin example has been updated to use data that does not cause errors.

With this document update, users can verify the behavior of the MCP server via stdin without using an MCP client.

README.md

@@ -121,11 +121,10 @@ transport.open
 
 You can run this script and then type in requests to the server at the command line.
 
-```
-$ ./stdio_server.rb
-{"jsonrpc":"2.0","id":"1","result":"pong"}
-{"jsonrpc":"2.0","id":"2","result":["ExampleTool"]}
-{"jsonrpc":"2.0","id":"3","result":["ExampleTool"]}
+```bash
+$ ./examples/stdio_server.rb
+{"jsonrpc":"2.0","id":"1","method":"ping"}
+{"jsonrpc":"2.0","id":"2","method":"tools/list"}
 ```
 
 ## Configuration