examples/config.rs: potential API key exposure through Debug output

[braveocheretovych]

examples/config.rs:12 prints the full Config struct via {config:#?}

Since url::Url serializes the complete URL string in its Debug output, fields like execution_rpc and fallback may expose API keys from the URL(e.g. https://eth-mainnet.g.alchemy.com/v2/SECRET_KEY) in stdout or CI logs.

Fix: replace with a non-sensitive message.