Fix typo: "an uint256" -> "a uint256" (#1658)
Fix typo: "an uint256" -> "a uint256" (#1658)

Using "a" instead of "an" makes this consistent with the comment on `allowance`.
OpenZeppelin/zep-solidity,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/zeppelin-solidity,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/zeppelin-solidity,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/zep-solidity