input: Write a python function to filter words by their length.
  assert filter_by_length(["apple", "banana", "cherry", "date", "fig", "grape"], 5) == ["apple"]
  assert filter_by_length(["one", "two", "three", "four", "five"], 3) == ["one", "two"]
  assert filter_by_length(["java", "python", "c", "ruby", "perl"], 4) == ["java", "ruby", "perl"]
output: def filter_by_length(words, length):
      return [word for word in words if len(word) == length]