docs/git-submodule: fix unbalanced quote

docs/git-submodule: fix unbalanced quote

The documentation gives an example of the submodule foreach
command that uses both backticks and single-quotes. We stick
the whole thing inside "+" markers to make it monospace, but
the inside punctuation still needs escaping. We handle the
backticks with "{backtick}", and use backslash-escaping for
the single-quotes.

But we missed the escaping on the second quote. Fortunately,
asciidoc renders this unbalanced quote as we want (showing
the quote), but asciidoctor does not. We could fix it by
adding the missing backslash.

However, let's take a step back. Even when rendered
correctly, it's hard to read a long command stuck into the
middle of a paragraph, and the important punctuation is hard
to notice. Let's instead bump it into its own single-line
code block. That makes both the source and the rendered
result more readable, and as a bonus we don't have to worry
about quoting at all.

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