printf: use plain text help headings

[nagendramohan]

printf --help printed raw Markdown headings (### ESCAPE SEQUENCES, #### SUBSTITUTION QUICK REFERENCE, ...) leaked from the after-help text, which the terminal shows verbatim.

Convert the Markdown headings to plain-text headings ending in a colon, matching the fix already applied to cut and dd in #13982. Updates both the en-US and fr-FR locale strings.

Fixes #13988

[codspeed-hq]

Merging this PR will improve performance by 4.9%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 356 untouched benchmarks
⏩ 50 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	numfmt_stream_to_si_precision	348.8 ms	332.5 ms	+4.9%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing nagendramohan:printf-plain-help-headings-13988 (ca2867d) with main (ec8adb5)}

Footnotes

1. 50 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.