Updates CollectionInterface::reduce() doc block, changing return type…

… hint to 'mixed'

src/Collection/CollectionInterface.php

@@ -183,7 +183,7 @@ public function map(callable $c);
      *
      * @param callable $c The callback function to be called
      * @param mixed $zero The state of reduction
-     * @return void
+     * @return mixed
      */
     public function reduce(callable $c, $zero = null);