jshn: fix missing usage for -p and -o arguments
Add missing usage hints for -p and -o arguments.
Fixes: e16fa068a573 ("jshn: add support for namespaces")
Fixes: eb30a03048f8 ("libubox, jshn: add option to write output to a file")
Signed-off-by: Petr Štetiar <ynezz@true.cz>