Issue #2939724 by ryanhayes, joachim, Samvel, andrewmacpherson, sacha@wellborn.co.uk, Adam_Moulsdale, rachel_norfolk: wrong variable name in sample code

(cherry picked from commit 441f198ba3)

core/lib/Drupal/Core/Database/Query/SelectInterface.php

@@ -95,7 +95,7 @@ interface SelectInterface extends ConditionInterface, AlterableInterface, Extend
    * Note that this method must be called by reference as well:
    *
    * @code
-   * $fields =& $query->getTables();
+   * $tables =& $query->getTables();
    * @endcode
    *
    * @return