vesting: more corrections on comments

contracts/token/VestedToken.sol

@@ -131,7 +131,7 @@ contract VestedToken is StandardToken, LimitedTransferToken {
    * @dev Calculate amount of vested tokens at a specific time
    * @param tokens uint256 The amount of tokens granted
    * @param time uint64 The time to be checked
-   * @param start uint64 A time representing the begining of the grant
+   * @param start uint64 The time representing the beginning of the grant
    * @param cliff uint64 The cliff period, the period until which nothing can be vested
    * @param vesting uint64 The vesting period
    * @return An uint256 representing the amount of vested tokens of a specific grant
@@ -166,7 +166,7 @@ contract VestedToken is StandardToken, LimitedTransferToken {
       // As before cliff the shortcut returns 0, we can use just calculate a value
       // in the vesting rect (as shown in above's figure)
 
-      // vestedTokens = tokens * (time - start) / (vesting - start)
+      // vestedTokens = (tokens * (time - start)) / (vesting - start)
       uint256 vestedTokens = SafeMath.div(
                                     SafeMath.mul(
                                       tokens,