[3.13] Docs: Fix indentation in `slice` class of `functions.rst` (GH-134393) (GH-135949)

Docs: Fix indentation in `slice` class of `functions.rst` (GH-134393)

Paragraph should not be under `slice.step`. It applies to the whole class.

(cherry picked from commit 6227662ff3bf838d31e9441eda935d24733d705a)

Co-authored-by: Rob Reynolds <13379223+reynoldsnlp@users.noreply.github.com>