docs(api): fix tmpfile() example to show matching output filename

[piy-ushk]

Fixes #1469

Corrects the inline comment in docs/api.md under the tmpfile() section.

The example showed tmpfile('f2.txt') resulting in a path ending in foo.txt, which doesn't match the passed argument. Under the hood, tempfile(name) returns path.join(tempdir(), name), so the returned path should end in f2.txt. This PR fixes the comment to accurately reflect the implementation.

import { tmpfile } from 'zx'

const f2 = tmpfile('f2.txt')  // -> /os/based/tmp/zx-1ra1iofojgg/f2.txt