experimental[patch]: Fix LLM graph transformer default prompt (#18856)

Some LLMs do not allow multiple user messages in sequence.

libs/experimental/langchain_experimental/graph_transformers/llm.py

@@ -52,14 +52,12 @@ default_prompt = ChatPromptTemplate.from_messages(
         (
             "human",
             (
+                "Tip: Make sure to answer in the correct format and do "
+                "not include any explanations. "
                 "Use the given format to extract information from the "
                 "following input: {input}"
             ),
         ),
-        (
-            "human",
-            "Tip: Make sure to answer in the correct format and do not include any ",
-        ),
     ]
 )