input: Write a function to replace all occurrences of a given character in a string with the provided replacement character. Return the modified string.
  
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Hello World!", 'o', 'Y')=="HellY WYrld!"
  assert replace_all_occurrences("Hello World!", ' ', '_')=="Hello_World!"
  assert replace_all_occurrences("Hello World!", 'Z', 'J')=="Hello World!"
  assert replace_all_occurrences("", 'a', 'b')==""
output: def replace_all_occurrences(s, char_to_replace, replacement_char):
      return s.replace(char_to_replace, replacement_char)