input: Write a python function to replace all occurrences of 'a' with 'A' in a string.
  assert change_a_to_A('apple')=='Apple'
  assert change_a_to_A('banana')=='bAnAnA'
  assert change_a_to_A('grape')=='grApe'
output: def change_a_to_A(s):
      return s.replace('a', 'A')