Detect RSS from the Content-Type of the HEAD response

The strpos arguments were reversed, so a HEAD response with an
application/rss+xml Content-Type was never reported as rss.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/p3k/WebSub/Client.php

@@ -56,7 +56,7 @@ class Client {
         if(strpos($content_type, 'text/html') !== false) {
           $type = $http['type'] = 'html';
         } else if(strpos($content_type, 'xml') !== false) {
-          if(strpos('rss', $content_type) !== false) {
+          if(strpos($content_type, 'rss') !== false) {
             $type = $http['type'] = 'rss';
           } else if(strpos($content_type, 'atom') !== false) {
             $type = $http['type'] = 'atom';