docs: change the pattern of FHEVMConfig

[poppyseedDev]

import "./FHEVMConfig.sol";

/// @title PaymentLimit
/// @notice A contract to demonstrate FHE gas limits in different scenarios
contract PaymentLimit {
    /// @notice Constructor that sets up FHE configuration and deposits initial value
    /// @dev Payable to allow initial deposit
    constructor() {
        FHE.setCoprocessor(FHEVMConfig.defaultConfig());
    }