Fix documentation on `LibWithdrawalConfig`

[guidanoli]

📚 Context

rollups-contracts/src/library/LibWithdrawalConfig.sol

Line 20 in 67a3487

// The addition below cannot overflow because `type(uint8).max + 1 <= type(uint256).max`.

Thanks @lynoferraz for spotting this typo!

✔️ Solution

type(uint8).max --> type(uint64).max