input: Write a python function to replace all occurrences of a given character in a string with another character.
  assert replace_char("This is a sentence with @ symbols.", '@', '#') == "This is a sentence with # symbols."
  assert replace_char("Hello@World@Python", '@', '*') == "Hello*World*Python"
  assert replace_char("No special character", '@', '#') == "No special character"
output: def replace_char(input_string, target_char, replacement_char):
      return input_string.replace(target_char, replacement_char)