Fix README snippet typo
Fix README snippet typo

(cherry picked from commit 0b8296e52596db8ef97f313fe3507cac8fbbcde3)

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