docs(guide/concepts): wording change

... or when working with --> a <-- third-party library callbacks.
... or when working with third-party library callbacks.

Author: brent-morrow

docs/content/guide/concepts.ngdoc

@@ -74,7 +74,7 @@ applied in Angular execution context will benefit from Angular data-binding, exc
 property watching, etc... You can also use $apply() to enter Angular execution context from JavaScript. Keep in
 mind that in most places (controllers, services) $apply has already been called for you by the
 directive which is handling the event. An explicit call to $apply is needed only when
-implementing custom event callbacks, or when working with a third-party library callbacks.
+implementing custom event callbacks, or when working with third-party library callbacks.
 
   1. Enter Angular execution context by calling {@link guide/scope scope}`.`{@link
      api/ng.$rootScope.Scope#$apply $apply}`(stimulusFn)`. Where `stimulusFn` is