gh-118476: update the example implementation for islice in docs

[FHTMitchell]

• The previous version was needlessly complicated
• It was also wrong since it overwrote stop == 0 to maxsize so

list(islice([1,2,3], 0))

looked like it returned [1,2,3] rather than [].

Fixes #118476

---

📚 Documentation preview 📚: https://cpython-previews--118466.org.readthedocs.build/

• Issue: Incorrect and overly-complicated islice example implementation in docs #118476

[ghost]

All commit authors signed the Contributor License Agreement.

[nineteendo]

Is it worth adding this comment here?

Suggested change

	for i, element in enumerate(it, start):
	if (i - start) % step == 0:
	for i, element in enumerate(it, start):
	# Consume to *stop*.
	if (i - start) % step == 0:

[rhettinger]

Closing this because a much improved version was found by Stefan Pochmann.