man: Fix typo and use $() for make expressions

Due to the typo, none of the .xml files would end up in the release
tarball and cause make distcheck as well as builds from the tarball to
fail.

Using $() isn't strictly necessary but other variables and expressions
use that variant already so it makes the usage consistent.

Reviewed-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>