input: Write a python function to replace spaces in a string with dots.
  assert fill_blanks("Python Programming") == "Python.Programming"
  assert fill_blanks("Hello World") == "Hello.World"
  assert fill_blanks(" ") == "."
  assert fill_blanks("NoSpaceHere") == "NoSpaceHere"
output: def fill_blanks(s):
      return s.replace(" ", ".")