docs(aio): remove services plurality (angular#20696)

remove services plurality for the sentence formation to be proper

PR Close angular#20696

Author: A7xSV

aio/content/tutorial/toh-pt4.md

@@ -3,7 +3,7 @@
 The Tour of Heroes `HeroesComponent` is currently getting and displaying fake data.
 
 After the refactoring in this tutorial, `HeroesComponent` will be lean and focused on supporting the view.
-It will also be easier to unit-test with a mock services.
+It will also be easier to unit-test with a mock service.
 
 ## Why services