StakeRegistry: fix inaccurate naming

[wadealexc]

From Dedaub audit, item A5:

The name of the function (and its argument) below is slightly misleading:
StakeRegistry::_validateOperatorStakeUpdateAtBlockNumber:445

function _validateOperatorStakeUpdateAtBlockNumber(
   	StakeUpdate memory operatorStakeUpdate,
    	uint32 blockNumber
) internal pure { … }

The function is used to validate StakeUpdate entries not just of the operatorStakeHistory mapping but also of the totalStakeHistory mapping.