Fix typo in Crowdsale.sol (#1035)

contracts/crowdsale/Crowdsale.sol

@@ -14,7 +14,7 @@ import "../token/ERC20/SafeERC20.sol";
  * The external interface represents the basic interface for purchasing tokens, and conform
  * the base architecture for crowdsales. They are *not* intended to be modified / overriden.
  * The internal interface conforms the extensible and modifiable surface of crowdsales. Override
- * the methods to add functionality. Consider using 'super' where appropiate to concatenate
+ * the methods to add functionality. Consider using 'super' where appropriate to concatenate
  * behavior.
  */
 contract Crowdsale {