Update demos-php/inc/body-corps-2a.php

My solution to bug as described here: https://github.com/wet-boew/wet-boew-php/issues/15

Seems like `$_TEMPLATE['foot_' . $_PAGE['lang']]` does not exist, (nor does any `$_TEMPLATE` arrays exist), resulting in an empty h2 element.

I fixed this myself simply by opening "php-demo/inc/body-corps-2a.php" and replacing:

`$_TEMPLATE['foot_'`

with:

`$_UNKNOWN['gcwu_foot_'`