In the series.Slice function, the k value should be allowed to be equal to s.len()

[l-grebe]

i see the code is:

	if j > k || j < 0 || k >= s.Len() {
		empty := s.Empty()
		empty.Err = fmt.Errorf("slice index out of bounds")
		return empty
	}

but maybe the following is right：

	if j > k || j < 0 || k > s.Len() {
		empty := s.Empty()
		empty.Err = fmt.Errorf("slice index out of bounds")
		return empty
	}