Update contract-calls.mdx

Author: kilavvy

pages/stack/interop/tutorials/contract-calls.mdx

@@ -154,7 +154,7 @@ function receiveBall(PingPongBall memory _ball) onlyCrossDomainCallback() extern
 ```
 
 *   The handler simply stores reference to the received ball
-*   The handler can only be invokable by the cross chain messenger
+*   The handler can only be invocable by the cross chain messenger
 *   Since the contract is self-referential, the cross chain sender must be the same contract address
 
 ### Hit the ball cross-chain