Remove default parameter in updateInstructions()

Some browsers do not support the use of default parameters. Moving the
logic that sets `step` to null if undefined into the function body
allows those browsers to render the tutorial correctly.

www/tutorial/app/app.component.ts

@@ -68,7 +68,7 @@ export class AppComponent implements OnInit {
   }
 
   // called when tutorial commands need to be displayed
-  updateInstructions(step = null) {
+  updateInstructions(step?) {
     // if step is given, we calculate the new index
     let totalSteps = this.instructionsArray.length - 1;
     switch(step) {