You are using PHP tag inside echo statement.
Try this:
echo '<a href="'.wp_logout_url().'" title="Logout">Logout</a>';
If you want to redirect to any specific url then you can pass it as argument of wp_logout_url.
Check official documentation of wp_logout_url.
Note :
It is recommended to return value from the shortcode function rather than directly displaying it. Try return instead of printing output by echo.