Update building-simple-cmdset-with-dialog-api.md (SharePoint#1405)

As far as I know the IListViewCommandSetExecuteEventParameters interface doesn't have a commandId property. It should be the itemId property:
https://docs.microsoft.com/en-us/javascript/api/sp-listview-extensibility/ilistviewcommandsetexecuteeventparameters

Author: expiscornovus

docs/spfx/extensions/get-started/building-simple-cmdset-with-dialog-api.md

@@ -221,7 +221,7 @@ The default solution takes advantage of a new Dialog API, which can be used to s
     ```typescript
       @override
       public onExecute(event: IListViewCommandSetExecuteEventParameters): void {
-        switch (event.commandId) {
+        switch (event.itemId) {
           case 'COMMAND_1':
             Dialog.alert(`Clicked ${strings.Command1}`);
             break;
@@ -387,4 +387,4 @@ Now you are ready to deploy the solution to a SharePoint site and have the `Cust
 ## See also
 
 - [Build your first Field Customizer extension](./building-simple-field-customizer.md)
-- [Overview of SharePoint Framework Extensions](../overview-extensions.md)
+- [Overview of SharePoint Framework Extensions](../overview-extensions.md)