fix openai example code error (#283)

Author: kelvin-lc

integrations/openai.md

@@ -105,7 +105,7 @@ pipe = Pipeline()
 
 pipe.add_component("retriever", InMemoryBM25Retriever(document_store=docstore))
 pipe.add_component("prompt_builder", PromptBuilder(template=template))
-pipe.add_component("llm", OpenAIGenerator(api_key=Secret.from_token("YOUR_OPENAI_API_KEY"))
+pipe.add_component("llm", OpenAIGenerator(api_key=Secret.from_token("YOUR_OPENAI_API_KEY")))
 pipe.connect("retriever", "prompt_builder.documents")
 pipe.connect("prompt_builder", "llm")