[refurb] Make example error out-of-the-box (FURB122)

[MeGaGiGaGon]

Summary

Part of #18972

This PR makes for-loop-writes (FURB122)'s example error out-of-the-box. I also had to re-name the second case's variables to get both to raise at the same time, I suspect because of limitations in ruff's current semantic model. New names subject to bikeshedding, I just went with the least effort _b for binary suffix.

Old example

with Path("file").open("w") as f:
    for line in lines:
        f.write(line)

with Path("file").open("wb") as f:
    for line in lines:
        f.write(line.encode())

New example

from pathlib import Path

with Path("file").open("w") as f:
    for line in lines:
        f.write(line)

with Path("file").open("wb") as f_b:
    for line_b in lines_b:
        f_b.write(line_b.encode())

The "Use instead" section was also modified similarly.

Test Plan

N/A, no functionality/tests affected

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[dylwil3]

LGTM thank you!