input: Write a python function to classify sentiment based on keywords 'positive', 'negative', and 'neutral' in a string.
  assert classify_sentiment("This is a positive experience.") == 'Positive'
  assert classify_sentiment("This has a negative impact.") == 'Negative'
  assert classify_sentiment("I feel neutral about this.") == 'Neutral'
  assert classify_sentiment("This is an amazing service.") == 'Unknown'
output: def classify_sentiment(text):
      text = text.lower()
      if 'positive' in text:
          return 'Positive'
      elif 'negative' in text:
          return 'Negative'
      elif 'neutral' in text:
          return 'Neutral'
      else:
          return 'Unknown'