docs: Fix typo in singleton-services.md page (angular#32330)

There is a typo in the first sentence of the singleton-service.md documentation page.
PR Close angular#32330

aio/content/guide/singleton-services.md

@@ -1,6 +1,6 @@
 # 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.