message: docs: Fix typo in singleton-services.md page
  There is a typo in the first sentence of the singleton-service.md documentation page.
  PR Close
diff: # Singleton services
  -A singleton service is a service for which only once instance exists in an app.
  +A singleton service is a service for which only one instance exists in an app.
  For a sample app using the app-wide singleton service that this page describes, see the
  <live-example name="ngmodules"></live-example> showcasing all the documented features of NgModules.
  