PHPDocs: Correction in inline docs for last_query

[nextendweb]

In the file wp-includes/wp-db.php the $last_query property described as array in PHPDoc while it is a string.

Original:

<?php
        /**
         * Last query made
         *
         * @since 0.71
         * @var array
         */
        var $last_query;

Updated:

<?php
        /**
         * Last query made
         *
         * @since 0.71
         * @var string
         */
        var $last_query;

[SergeyBiryukov]

In 45430:

Docs: Correct @type annotation for wpdb::$last_query.

Props nextendweb, mukesh27.
Fixes #47095.