Update simple_server.dart (#187)

Make the example `simple_server.dart` work with Claude Desktop

<img width="549" alt="Screenshot 2025-06-25 at 6 54 11 pm" src="https://github.com/user-attachments/assets/dc796e1e-d0a5-44cf-b7cb-8fdaca257bfe" />

Author: domesticmouse

pkgs/dart_mcp/example/simple_server.dart

@@ -39,7 +39,7 @@ base class DartMCPServer extends MCPServer with ToolsSupport {
   @override
   FutureOr<InitializeResult> initialize(InitializeRequest request) {
     registerTool(
-      Tool(name: 'hello world', inputSchema: ObjectSchema()),
+      Tool(name: 'hello_world', inputSchema: ObjectSchema()),
       (_) => CallToolResult(content: [TextContent(text: 'hello world!')]),
     );
     return super.initialize(request);