input: Write a function that computes the harmonic series sum up to n-1.
  assert harmonic_series_sum(7) == 2.5928571428571425
  assert harmonic_series_sum(4) == 2.083333333333333
  assert harmonic_series_sum(19) == 3.547739657143682
output: def harmonic_series_sum(n):
      return sum(1/i for i in range(1, n))