docs(tutorial/step-8): link to Protractor rather than ngScenario

Angular test runner API has been deprecated, Protractor is advised:
http://docs.angularjs.org/guide/e2e-testing
Link is updated to direct reader to Protractor API.

Closes #7001

Author: yilinglu

docs/content/tutorial/step_08.ngdoc

@@ -184,8 +184,8 @@ You can now rerun `npm run protractor` to see the tests run.
 
 # Experiments
 
-* Using the {@link guide/dev_guide.e2e-testing Angular's end-to-end test runner API}, write a test
-that verifies that we display 4 thumbnail images on the Nexus S details page.
+* Using the {@link https://github.com/angular/protractor/blob/master/docs/api.md Protractor API},
+write a test that verifies that we display 4 thumbnail images on the Nexus S details page.
 
 
 # Summary