Fix a formatting error in git-merge.txt

Fix a formatting error in git-merge.txt

Inside an element of an enumerated list, the second and subsequent
paragraphs need to lose their indent and have to be strung together with a
line with a single '+' on it instead.  Otherwise the lines below are shown
in typewriter face, which just looks wrong.

Signed-off-by: Nathan W. Panike <nathan.panike@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>