docs: add clarification in step 2 of the tutorial

Author: naomiblack

docs/content/tutorial/step_02.ngdoc

@@ -206,7 +206,7 @@ is located at `./config/karma.conf.js`).
 
   Yay! The test passed! Or not...
 
-4. To rerun the tests, just change any of the source or test files. Karma will notice the change
+4. To rerun the tests, just change any of the source or test .js files. Karma will notice the change
   and will rerun the tests for you. Now isn't that sweet?
 
 # Experiments