input: Write a python function to replace a pattern in a string with a replacement string.
  assert replacePattern("abcabcabc", "abc", "xyz") == "xyzxyzxyz"
  assert replacePattern("hello world", "world", "there") == "hello there"
  assert replacePattern("123123123", "123", "456") == "456456456"
output: def replacePattern(s, pattern, replacement):
      return s.replace(pattern, replacement)