git-submodule documentation: fix foreach example

Backtick and apostrophe are asciidoc markup, so they should be escaped
in order to get the expected result in the rendered manual page.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/asciidoc.conf

@@ -17,6 +17,7 @@ caret=^
 startsb=[
 endsb=]
 tilde=~
+backtick=`
 
 ifdef::backend-docbook[]
 [linkgit-inlinemacro]

Documentation/git-submodule.txt

@@ -137,8 +137,9 @@ foreach::
 	the processing to terminate. This can be overridden by adding '|| :'
 	to the end of the command.
 +
-As an example, "git submodule foreach 'echo $path `git rev-parse HEAD`' will
-show the path and currently checked out commit for each submodule.
+As an example, +git submodule foreach \'echo $path {backtick}git
+rev-parse HEAD{backtick}'+ will show the path and currently checked out
+commit for each submodule.
 
 sync::
 	Synchronizes submodules' remote URL configuration setting