docs: update hierarchical-dependency-injection.md

VillainsService highly likely misspelled and then written as VillainService with 's' missing

Author: lyutails

adev/src/content/guide/di/hierarchical-dependency-injection.md

@@ -959,8 +959,8 @@ export class VillainsList {}
 
 By providing `VillainsService` in the `VillainsList` metadata and nowhere else, the service becomes available only in the `VillainsList` and its subcomponent tree.
 
-`VillainService` is a singleton with respect to `VillainsList` because that is where it is declared.
-As long as `VillainsList` does not get destroyed it will be the same instance of `VillainService` but if there are multiple instances of `VillainsList`, then each instance of `VillainsList` will have its own instance of `VillainService`.
+`VillainsService` is a singleton with respect to `VillainsList` because that is where it is declared.
+As long as `VillainsList` does not get destroyed it will be the same instance of `VillainsService` but if there are multiple instances of `VillainsList`, then each instance of `VillainsList` will have its own instance of `VillainsService`.
 
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