git-format-patch(1): fix stray \ in output

In listing blocks (set off by rows of dashes), the usual
formatting characters of asciidoc are instead rendered verbatim.
When the escaped double-hyphen of olden days is moved into such a
block along with other formatting improvements, it becomes
backslash-dash-dash.

So we remove the backslash.
Signed-off-by: NJonathan Nieder <jrnieder@uchicago.edu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-format-patch.txt

@@ -206,7 +206,7 @@ For each commit a separate file is created in the current directory.
 project:
 +
 ------------
-$ git format-patch \--root origin
+$ git format-patch --root origin
 ------------
 
 * The same as the previous one: