input: Write a function to replace all occurrences of a specified character in a given string with another specified replacement character. If the specified character is not found in the string, return the original string.
  assert replace_all_occurrences("Hello World!", 'l', 'X')== "HeXXo WorXd!"
  assert replace_all_occurrences("Python Programming", 'o', '0')== "Pyth0n Pr0gramming"
  assert replace_all_occurrences("Hello World!", 'z', 'Y')== "Hello World!"
  assert replace_all_occurrences("", 'a', 'b')== ""
  assert replace_all_occurrences("This is a test", ' ', '_')== "This_is_a_test"
output: def replace_all_occurrences(s, char, replacement):
      return s.replace(char, replacement)