Fix wrong type hint (#678)

`SimplePie_Cache` also exists but is something else.
This created errors in IDE.
For instance, `SimplePie_Cache::load()` does not exist, but `SimplePie_Cache_Base::load()` does.

Downtream PR: FreshRSS/FreshRSS#3578

library/SimplePie.php

@@ -1521,7 +1521,7 @@ public function init()
 	 * 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)