merge-file doc: set conflict-marker-size attribute

merge-file doc: set conflict-marker-size attribute

When committing a conflict resolution for a merge containing
1f010d6bdf7 (doc: use .adoc extension for AsciiDoc files, 2025-01-20)
my pre-commit hook failed because "git diff --check" thought there was
a left over conflict marker in "merge-file.adoc". Fix this by setting
the "conflict-marker-size" attribute as we do for all the other
documentation files that contain example conflict markers.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>