Fix wrong SimplePie type hint (#3578)

`SimplePie_Cache` also exists but is something else.

lib/SimplePie/SimplePie.php

@@ -1622,7 +1622,7 @@ class SimplePie
 	 * Fetch the data via SimplePie_File
 	 *
 	 * If the data is already cached, attempt to fetch it from there instead
-	 * @param SimplePie_Cache|false $cache Cache handler, or false to not load from the cache
+	 * @param SimplePie_Cache_Base|false $cache Cache handler, or false to not load from the cache
 	 * @return array|true Returns true if the data was loaded from the cache, or an array of HTTP headers and sniffed type
 	 */
 	protected function fetch_data(&$cache)