docs: fix README.md for Hoodi testnet

[letmehateu]

Summary

Replaces Holesky RPC endpoints with Hoodi-specific ones in the documentation example, as Holesky testnet support will be discontinued soon.

Changes

• Updated consensus RPC to Hoodi beacon API endpoint
• Updated execution RPC to Hoodi-specific endpoint
• Fixed links

Rationale

Holesky testnet is being deprecated, so examples should use the appropriate network-specific endpoints for Hoodi.