Change name of YellowClearing var to mixedcase

[nksazonov]

To comply with solhint rules.

From

struct IdentityPayload {
    YellowClearingBase YellowClearing;
    address participant;
    uint64 nonce;
}

To

struct IdentityPayload {
    YellowClearingBase yellowClearing;
     address participant;
     uint64 nonce;
}