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

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>