input: Write a function to replace a target substring with a replacement substring in a given input string.
  assert replace_string("Hello World!", "World", "Python") == "Hello Python!"
  assert replace_string("The quick brown fox", "quick", "slow") == "The slow brown fox"
  assert replace_string("Banana", "na", "ma") == "Bamama"
output: def replace_string(input_string, target_substring, replacement_substring):
      return input_string.replace(target_substring, replacement_substring)