input: Write a python function to combine a list of strings into a single string separated by spaces.
  assert combine_strings(["hello", "world"]) == "hello world"
  assert combine_strings(["this", "is", "a", "test"]) == "this is a test"
  assert combine_strings(["combine", "these", "strings"]) == "combine these strings"
output: def combine_strings(strings):
      return ' '.join(strings)