docs: fix tmpfile example comment

[islamhafez0]

What

Fixes #1469. The docs example for mpfile() shows the wrong output filename.

Why

mpfile('f2.txt') returns path.join(tempdir(), 'f2.txt') (see src/goods.ts), so the resulting path ends in f2.txt, not foo.txt. The inline comment was misleading.

Checklist

• Verified against src/goods.ts implementation

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[islamhafez0]

@googlebot I signed it!