doc/config/push: use longer "--" line for preformatted example

doc/config/push: use longer "--" line for preformatted example

The example for the push.pushOption config tries to create a
preformatted section, but uses only two dashes in its "--" line. In
AsciiDoc this is an "open block", with no type; the lines end up jumbled
because they're formatted as paragraphs. We need four or more dashes to
make it a "listing block" that will respect the linebreaks.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>