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

Using "a" instead of "an" makes this consistent with the comment on `allowance`.

contracts/token/ERC20/ERC20.sol

@@ -34,7 +34,7 @@ contract ERC20 is IERC20 {
     /**
      * @dev Gets the balance of the specified address.
      * @param owner The address to query the balance of.
-     * @return An uint256 representing the amount owned by the passed address.
+     * @return A uint256 representing the amount owned by the passed address.
      */
     function balanceOf(address owner) public view returns (uint256) {
         return _balances[owner];