Documentation/rev-list-options: wrap --date=<format> block with "--"

Documentation/rev-list-options: wrap --date=<format> block with "--"

Using "+" to continue multiple list items is more tedious and
error-prone than wrapping the entire block with "--" block markers.

When using asciidoctor, the list items after the --date=iso list items
are incorrectly formatted when using "+" continuation.  Use "--" block
markers to correctly format the block.

When using asciidoc there is no change in how the content is rendered.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>