docs(tutorial/step_03): add info about karma-ng-scenario plug-in
docs(tutorial/step_03): add info about karma-ng-scenario plug-in

The existing documentation for this step could have people find themselves
unable to run the `e2e-test.sh` script. This note added regarding
`karma-ng-scenario` will minimize their confusion and allow people to run
the script.

Closes #4033

ammula88/angular.js,ammula88/angular.js,ammula88/angular.js,ammula88/angular.js,ammula88/angular.js