ab3TracingCode: Fix typo Deciptor -> Descriptor

tutorials/ab3TracingCode.md

@@ -202,7 +202,7 @@ Recall from the User Guide that the `edit` command has the format: `edit INDEX [
 
 14. Stepping through the method shows that it calls `ArgumentTokenizer#tokenize()` and `ParserUtil#parseIndex()` to obtain the arguments and index required.
 
-15. The rest of the method seems to exhaustively check for the existence of each possible parameter of the `edit` command and store any possible changes in an `EditPersonDescriptor`. Recall that we can verify the contents of `editPersonDesciptor` through the 'Variables' window.<br>
+15. The rest of the method seems to exhaustively check for the existence of each possible parameter of the `edit` command and store any possible changes in an `EditPersonDescriptor`. Recall that we can verify the contents of `editPersonDescriptor` through the 'Variables' window.<br>
    ![EditCommand](images/tracing/EditCommand.png)
 
 16. As you just traced through some code involved in parsing a command, you can take a look at this class diagram to see where the various parsing-related classes you encountered fit into the design of the `Logic` component.