input: Write a function to count the frequency of each letter in a given string.
  assert letter_frequency("Hello World") == {'H': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'W': 1, 'r': 1, 'd': 1}
  assert letter_frequency("Python Programming") == {'P': 2, 'y': 1, 't': 1, 'h': 1, 'o': 2, 'n': 2, ' ': 1, 'r': 2, 'g': 2, 'a': 1, 'm': 2, 'i': 1}
  assert letter_frequency("Data Science") == {'D': 1, 'a': 2, 't': 1, ' ': 1, 'S': 1, 'c': 2, 'i': 1, 'e': 2, 'n': 1}
output: def letter_frequency(s):
      freq = {}
      for char in s:
          if char in freq:
              freq[char] += 1
          else:
              freq[char] = 1
      return freq