update libraries for langchain_community + llama3 changed from llama2 (#4174)

examples/langchain-python-rag-websummary/main.py

-from langchain.llms import Ollama
-from langchain.document_loaders import WebBaseLoader
+from langchain_community.llms import Ollama
+from langchain_community.document_loaders import WebBaseLoader
 from langchain.chains.summarize import load_summarize_chain
 
 loader = WebBaseLoader("https://ollama.com/blog/run-llama2-uncensored-locally")
 docs = loader.load()
 
-llm = Ollama(model="llama2")
+llm = Ollama(model="llama3")
 chain = load_summarize_chain(llm, chain_type="stuff")
 
-result = chain.run(docs)
+result = chain.invoke(docs) 
 print(result)