input: Write a function to filter a DataFrame based on a rating column.
  assert filter_by_rating(df, 4).equals(filtered_df)
  assert filter_by_rating(df, 5).equals(df[df['rating'] == 5])
  assert filter_by_rating(df, 3).equals(df[df['rating'] >= 3])
output: def filter_by_rating(df, rating):
      if rating == 5:
          return df[df['rating'] == 5]
      else:
          return df[df['rating'] >= rating]