input: Write a python function to replace a specified character in a string with another character.
  assert replace_char("Hello World", "l", "X") == "HeXXo WorXd"
  assert replace_char("banana", "a", "o") == "bonono"
  assert replace_char("test case", "e", "i") == "tist casi"
output: def replace_char(s, old_char, new_char):
      return s.replace(old_char, new_char)