[Doc] Use registered callable names in examples/README.md

[yuki3738]

Motivation and Context

The tools/call cURL example in examples/README.md uses "name": "ExampleTool", but a tool registered by class without an explicit tool_name gets a snake_case name derived from its class name (via StringUtils.handle_from_class_name), so the example fails with Tool not found: ExampleTool. The Tools/Prompts lists in the same file had the same mismatch (ExampleTool, ExamplePrompt, NotificationTool). All entries now use the registered callable names.

How Has This Been Tested?

Ran ruby examples/http_server.rb locally and exercised the cURL flow against it (SSE event payloads shown without the data: prefix):

## tools/list
{"jsonrpc":"2.0","id":2,"result":{"tools":[{"name":"example_tool",...},{"name":"echo",...}]}}

## tools/call with "name":"ExampleTool" (before)
{"jsonrpc":"2.0","id":3,"error":{"code":-32602,"message":"Invalid params","data":"Tool not found: ExampleTool"}}

## tools/call with "name":"example_tool" (after)
{"jsonrpc":"2.0","id":4,"result":{"content":[{"type":"text","text":"The sum of 5 and 3 is 8"}],"isError":false}}

## prompts/list
{"jsonrpc":"2.0","id":5,"result":{"prompts":[{"name":"example_prompt",...}]}}

The full unit suite and RuboCop pass locally.

Breaking Changes

None (documentation only).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added or updated documentation as needed

[yuki3738]

Merged upstream as modelcontextprotocol#437 (modelcontextprotocol#437). Closing this staging PR.