adjust link to point to v3.8 of behavior tree docs (#3851)

BT.CPP_v3 is used, thereby the correct docs should be linked

nav2_behavior_tree/README.md

@@ -63,4 +63,4 @@ The BehaviorTree engine has a run method that accepts an XML description of a BT
 
 See the code in the [BT Navigator](../nav2_bt_navigator/src/bt_navigator.cpp) for an example usage of the BehaviorTreeEngine.
 
-For more information about the behavior tree nodes that are available in the default BehaviorTreeCPP library, see documentation here: https://www.behaviortree.dev/docs/learn-the-basics/bt_basics/
+For more information about the behavior tree nodes that are available in the default BehaviorTreeCPP library, see documentation here: https://www.behaviortree.dev/docs/3.8/learn-the-basics/BT_basics