Documentation: fix header markup
Documentation: fix header markup

Asciidoctor is stricter than AsciiDoc when deciding if underlining
is a section title or the start of preformatted text.  Make the
length of the underlining match the text to ensure that it renders
correctly in all implementations.

Signed-off-by: Xue Fuqiao <71b52ec50d6675b30625f9b9f305490cfa824ff0@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git