WEB3-333: fix: unstable-history doctest

[Wollac]

The doctest gets included even though the corresponding method is behind a feature using #[stability::unstable(feature = "history")]. This leads to failing tests if the unstable-history feature is not enabled.
Fixed by adding a hidden #[cfg(feature = "unstable-history")] to the example, in the docs it still looks like the following:

let commitment_hash = B256::from_str("0x1234...")?;
let builder = EthEvmEnv::builder()
    .rpc(Url::parse("https:// ethereum-rpc. publicnode. com")?)
    .beacon_api(Url::parse("https:// ethereum-beacon-api. publicnode. com")?)
    .block_number(1_000_000);  // execute against historical state
let builder = builder.commitment_block_hash(commitment_hash); // commit to recent block
let env = builder.build().await?;