more php 8.x compatibility

system/libs/pot/OTS_SpellsList.php

@@ -308,7 +308,7 @@ class OTS_SpellsList implements IteratorAggregate, Countable
  * @since 0.1.5
  * @return AppendIterator Iterator for all spells.
  */
-    public function getIterator()
+    public function getIterator(): Traversable
     {
         $iterator = new AppendIterator();
         $iterator->append( new ArrayIterator($this->runes) );