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>

Documentation/config/push.txt

@@ -79,7 +79,7 @@ higher priority configuration file (e.g. `.git/config` in a
 repository) to clear the values inherited from a lower priority
 configuration files (e.g. `$HOME/.gitconfig`).
 +
---
+----
 
 Example:
 
@@ -96,7 +96,7 @@ repo/.git/config
 
 This will result in only b (a and c are cleared).
 
---
+----
 
 push.recurseSubmodules::
 	Make sure all submodule commits used by the revisions to be pushed