The .txt extensions were changed to .adoc in 1f010d6bdf (doc: use .adoc
extension for AsciiDoc files, 2025-01-20). This left broken links in
the generated howto-index.html.
Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>