update example of ConstitutionalChain.from_llm (#3630)

Example code was missing an argument and import. Fixed.

langchain/chains/constitutional_ai/base.py

@@ -18,14 +18,19 @@ class ConstitutionalChain(Chain):
 
             from langchain.llms import OpenAI
             from langchain.chains import LLMChain, ConstitutionalChain
+            from langchain.chains.constitutional_ai.models \
+                import ConstitutionalPrinciple
+
+            llm = OpenAI()
 
             qa_prompt = PromptTemplate(
                 template="Q: {question} A:",
                 input_variables=["question"],
             )
-            qa_chain = LLMChain(llm=OpenAI(), prompt=qa_prompt)
+            qa_chain = LLMChain(llm=llm, prompt=qa_prompt)
 
             constitutional_chain = ConstitutionalChain.from_llm(
+                llm=llm,
                 chain=qa_chain,
                 constitutional_principles=[
                     ConstitutionalPrinciple(