Update README.md. Fix vLLM code example.

README.md

@@ -158,7 +158,7 @@ prompts = [
     "What is 291 - 150?",
     "How much wood would a woodchuck chuck if a woodchuck could chuck wood?",
 ]
-prompt_template=f'''<|system|>
+prompt_template='''<|system|>
 {system_message}</s>
 <|user|>
 {prompt}</s>