xtensa: fix variants path in the Kconfig help

The directory in the path should be "variants" instead of "variant".

Fixes: 420ae951

 ("xtensa: simplify addition of new core variants")
Signed-off-by: Yujie Liu <yujie.liu@intel.com>
Message-Id: <20231129091337.2084747-1-yujie.liu@intel.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>