Fix documentation on Area::setCustomTemplate() method

The docs were giving the wrong example for how to pass in a custom template name (it should be 'templates/file_name.php', not just the template handle as was previously being indicated).
Also refactored the variable name from $temp to $view, so it matches the Block::display() method arg (thus giving another indication that you should pass in the relative path to the custom template file, and not just the handle).