Fixes typo in SafeMath subtraction docs (#806)

contracts/math/SafeMath.sol

@@ -30,7 +30,7 @@ library SafeMath {
   }
 
   /**
-  * @dev Substracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
+  * @dev Subtracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
   */
   function sub(uint256 a, uint256 b) internal pure returns (uint256) {
     assert(b <= a);