docs: correct the default ISO weekday in isoparse - #1630

Closes #1629.

Correct the isoparse docstring to say an ISO week without an explicit weekday defaults to 1, Monday. The previous wording said 0, although the parser uses dayno = 1 and rejects day 0. No parsing behavior changes.

Validation on Python 3.11.15:

• Existing ISO parser suite: 567 passed, 1 xfailed.
• Confirmed extended/basic week-only inputs match the explicit Monday form, including a week crossing the calendar-year boundary; day 0 still raises ValueError.
• Full Sphinx HTML build with -W passed; the rendered parser page contains the corrected default.
• git diff --check passed.

Prepared and validated with Codex.