input: Write a python function to concatenate a list of strings into a single string with spaces in between.
  assert strings_to_string(['Hello', 'world!', 'Welcome', 'to', 'the', 'programming', 'world!']) == 'Hello world! Welcome to the programming world! '
  assert strings_to_string(['Python', 'is', 'fun']) == 'Python is fun '
  assert strings_to_string(['I', 'love', 'coding']) == 'I love coding '
output: def strings_to_string(strings):
      return ' '.join(strings) + ' '