fix: Update the docstring for LangchainAgent

PiperOrigin-RevId: 681158401

Author: yeesian

vertexai/preview/reasoning_engines/templates/langchain.py

@@ -269,9 +269,8 @@ def _override_active_span_processor(
 class LangchainAgent:
  """A Langchain Agent.
 
- Reference:
- * Agent: https://python.langchain.com/docs/modules/agents/concepts
- * Memory: https://python.langchain.com/docs/expression_language/how_to/message_history
+ See https://cloud.google.com/vertex-ai/generative-ai/docs/reasoning-engine/develop
+ for details.
  """
 
  def __init__(