Include animation.length in Animation example

The length of an Animation isn't automatically set by adding keys, and
it must be set manually. The existing example only has keys up to 0.5s,
so the default value of 1.0s may be acceptable. However, this results in
unexpected behavior for anyone who makes an animation longer than 1.0s.
Include animation.length in the example because it's important.

Also, increase the key position to 2.0s so it won't be confusing that
the assignment is somewhat redundant.

(cherry picked from commit 698c67d540cddf38c9ad7e73e99721c245b93588)

doc/classes/Animation.xml

@@ -8,21 +8,23 @@
 		[codeblocks]
 		[gdscript]
 		# This creates an animation that makes the node "Enemy" move to the right by
-		# 100 pixels in 0.5 seconds.
+		# 100 pixels in 2.0 seconds.
 		var animation = Animation.new()
 		var track_index = animation.add_track(Animation.TYPE_VALUE)
 		animation.track_set_path(track_index, "Enemy:position:x")
 		animation.track_insert_key(track_index, 0.0, 0)
-		animation.track_insert_key(track_index, 0.5, 100)
+		animation.track_insert_key(track_index, 2.0, 100)
+		animation.length = 2.0
 		[/gdscript]
 		[csharp]
 		// This creates an animation that makes the node "Enemy" move to the right by
-		// 100 pixels in 0.5 seconds.
+		// 100 pixels in 2.0 seconds.
 		var animation = new Animation();
 		int trackIndex = animation.AddTrack(Animation.TrackType.Value);
 		animation.TrackSetPath(trackIndex, "Enemy:position:x");
 		animation.TrackInsertKey(trackIndex, 0.0f, 0);
-		animation.TrackInsertKey(trackIndex, 0.5f, 100);
+		animation.TrackInsertKey(trackIndex, 2.0f, 100);
+		animation.Length = 2.0f;
 		[/csharp]
 		[/codeblocks]
 		Animations are just data containers, and must be added to nodes such as an [AnimationPlayer] to be played back. Animation tracks have different types, each with its own set of dedicated methods. Check [enum TrackType] to see available types.