[Doc] Use callable name in HTTP server example tool list

[yuki3738]

Motivation and Context

The HTTP server example's "Tools" list in examples/README.md refers to the adder tool by its class name ExampleTool, while the sibling echo tool is listed by its callable name. The tool is registered from the ExampleTool class without an explicit tool_name, so its callable name is example_tool (derived via StringUtils.handle_from_class_name).

This lists it by the callable name, consistent with the echo entry beside it and with the tools/call examples further down.

How Has This Been Tested?

N/A (documentation only). The listed name now matches the tool registered by examples/http_server.rb.

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

[yuki3738]

Superseded by #7 (unified into a single doc PR with #5 — the related cURL example line).