Documentation/git-merge.txt: fix formatting of example block

You need at least four dashes in a line to have it recognized as listing
block delimiter by asciidoc.

Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'maint-1.8.2' into maint-1.8.3

* maint-1.8.2:
  Documentation/git-merge.txt: fix formatting of example block