tools: ynl-gen: fix comment about nested struct dict

The dict stores struct objects (of class Struct), not just
a trivial set with directions.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250429154704.2613851-2-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>