Update README.md. Fix vLLM code example.

Fix vLLM code example. Remove the f-string to allow it to work with the format method call on the subsequent list comprehension.

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>