doc/add: reformat --edit option

All of the other options in the list put short and long as
two separate headings.

We can also drop the backslashing of `--`. It isn't used
elsewhere and is unnecessary for modern asciidoc (plus it
confuses asciidoctor).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-add.txt

@@ -93,7 +93,8 @@ This effectively runs `add --interactive`, but bypasses the
 initial command menu and directly jumps to the `patch` subcommand.
 See ``Interactive mode'' for details.
 
--e, \--edit::
+-e::
+--edit::
 	Open the diff vs. the index in an editor and let the user
 	edit it.  After the editor was closed, adjust the hunk headers
 	and apply the patch to the index.