Documentation: fix formatting error in merge-options.txt

The first paragraph inside of a list item does not need a preceding line
consisting of a single '+', and in fact this causes the text to be
misrendered. Fix it.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/merge-options.txt

@@ -9,7 +9,6 @@ inspect and further tweak the merge result before committing.
 
 --edit::
 -e::
-+
 	Invoke editor before committing successful merge to further
 	edit the default merge message.