git-submodule.txt: quote usage in monospace, drop backslash

We tend to quote command line examples using `` to set them in a
monospace font. The immediate motivation for this patch is to get rid of
another instance of \--. As noted in the previous commits, \-- has a
tendency of rendering badly. Here, it renders ok (at least with
AsciiDoc 8.6.9 and Asciidoctor 1.5.4), but by getting rid of this
instance, we reduce the chances of \-- cropping up in places where it
matters more.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>

Documentation/git-submodule.txt

@@ -213,8 +213,8 @@ sync [--recursive] [--] [<path>...]::
 	submodule URLs change upstream and you need to update your local
 	repositories accordingly.
 +
-"git submodule sync" synchronizes all submodules while
-"git submodule sync \-- A" synchronizes submodule "A" only.
+`git submodule sync` synchronizes all submodules while
+`git submodule sync -- A` synchronizes submodule "A" only.
 +
 If `--recursive` is specified, this command will recurse into the
 registered submodules, and sync any nested submodules within.