Add an example to make the prompt more robust (#5291)

# Add example to LLMMath to help with power operator

Add example to LLMMath that helps the model to interpret `^` as the power operator rather than the python xor operator.

langchain/chains/llm_math/prompt.py

@@ -25,6 +25,16 @@ Question: What is 37593 * 67?
 ```
 Answer: 2518731
 
+Question: 37593^(1/5)
+```text
+37593**(1/5)
+```
+...numexpr.evaluate("37593**(1/5)")...
+```output
+8.222831614237718
+```
+Answer: 8.222831614237718
+
 Question: {question}
 """