added a span within query

src/Query/Span/SpanWithinQuery.php

+<?php
+
+/*
+ * This file is part of the ONGR package.
+ *
+ * (c) NFQ Technologies UAB <info@nfq.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace ONGR\ElasticsearchDSL\Query\Span;
+
+/**
+ * Elasticsearch span within query.
+ *
+ * @link https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-span-within-query.html
+ */
+class SpanWithinQuery extends SpanContainingQuery
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function getType()
+    {
+        return 'span_within';
+    }
+}