convert legacy ChatComplete syntax

convert legacy ChatComplete syntax.txt

  # before 
  completions = openai.ChatCompletion.create(
    model="gpt-3.5-turbo",
    messages=[
      {"role": "system", "content": prompt},
    ],
    temperature=0.4,
    max_tokens=999,
  )
  res = completions['choices'][0]['message']['content']
  
  # after
  completions = client.chat.completions.create(
    model="gpt-3.5-turbo",
    messages=[
      {"role": "system", "content": prompt},
    ],
    temperature=0.4,
    max_tokens=999,
  )
  res = completions.choices[0].message.content