Improve Navigator documentation (#29540)

packages/flutter/lib/src/widgets/navigator.dart

@@ -564,7 +564,7 @@ class NavigatorObserver {
 ///
 /// The page route is built in two parts, the "page" and the
 /// "transitions". The page becomes a descendant of the child passed to
-/// the `buildTransitions` method. Typically the page is only built once,
+/// the `transitionsBuilder` function. Typically the page is only built once,
 /// because it doesn't depend on its animation parameters (elided with `_`
 /// and `__` in this example). The transition is built on every frame
 /// for its duration.