Close dataframe column names are being treated as one by the LLM (#3611)

We are sending sample dataframe to LLM with df.head().
If the column names are close by, LLM treats two columns names as one,
returning incorrect results.


![image](https://user-images.githubusercontent.com/4707543/234678692-97851fa0-9e12-44db-92ec-9ad9f3545ae2.png)

In the above case the LLM uses **Org Week** as the column name instead
of **Week** if asked about a specific week.

Returning head() as a markdown separates out the columns names and thus
using correct column name.


![image](https://user-images.githubusercontent.com/4707543/234678945-c6d7b218-143e-4e70-9e17-77dc64841a49.png)

langchain/agents/agent_toolkits/pandas/base.py

@@ -35,7 +35,7 @@ def create_pandas_dataframe_agent(
     prompt = ZeroShotAgent.create_prompt(
         tools, prefix=prefix, suffix=suffix, input_variables=input_variables
     )
-    partial_prompt = prompt.partial(df=str(df.head()))
+    partial_prompt = prompt.partial(df=str(df.head().to_markdown()))
     llm_chain = LLMChain(
         llm=llm,
         prompt=partial_prompt,