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

[yuki3738]

Motivation and Context

examples/README.md referred to three example components by their Ruby class names, while the server examples register them via class registration without an explicit tool_name / prompt_name. Their callable names exposed over JSON-RPC are therefore derived to the snake_case form via StringUtils.handle_from_class_name (used by both MCP::Tool#name_value and MCP::Prompt#name_value).

• The tools/call cURL example for the HTTP server used "name": "ExampleTool", but the registered tool is example_tool, so it would fail with Tool not found: ExampleTool (-32602 Invalid params).
• The HTTP Server "Tools" list and "Prompts" list referred to ExampleTool and ExamplePrompt. The sibling echo tool is already listed by its callable name.
• The Streamable HTTP Server "Available Tools" list referred to NotificationTool, while the same section's cURL example already uses notification_tool.

The list and cURL entries now consistently use the callable names (example_tool, example_prompt, notification_tool).

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",...}]}}

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
• I have added or updated documentation as needed