By default, Relevanssi does not index the post slug. It is, however, an easy thing to fix using the relevanssi_content_to_index filter hook. This function will read the post slug, replace the hyphens with spaces (so that indexing-post-slugs becomes “indexing post slugs”) and add the slug to the post content.
Add this function to your site and rebuild the index to see it in effect.
add_filter( 'relevanssi_content_to_index', 'rlv_index_slug', 10, 2 );
function rlv_index_slug( $content, $post ) {
$slug_with_spaces = str_replace( '-', ' ', $post->post_name );
return $content . ' ' . $slug_with_spaces;
}