docs: correct tmpfile example output

[KirtiRamchandani]

Problem

The tmpfile('f2.txt') example in docs/api.md shows a returned path ending in foo.txt, which does not match the argument.

Root cause

The example comment appears to have been copied from the tmpdir('foo') example above it, while tmpfile(name) returns a path using the provided file name.

Solution

Update the example output to end in f2.txt.

Tests run

• npm exec --yes prettier -- --check docs/api.md
• git diff --check

Fixes #1469

[KirtiRamchandani]

Closing this since there are already earlier open PRs covering the same one-line docs fix (#1470 and #1474).