Update Counter.sol (#1917)
Update Counter.sol (#1917)

* Update Counter.sol

Added a comment in increment to make this more visible to not use SafeMath here.

* Update Counters.sol

OpenZeppelin/openzeppelin-contracts,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/zep-solidity,OpenZeppelin/zeppelin-solidity,OpenZeppelin/zep-solidity,OpenZeppelin/zeppelin-solidity