Fixed footer.php for greaves

Outdated call to function Admin URL generate an error message:

UNCAUGHT EXCEPTION
Call to undefined function admin_url()

ORIGIN

themes/greaves/footer.php on line 14

TRACE

#0 [internal function]: System\Error::shutdown()
#1 {main}

Replacing on line 14 of footer.php:

<?php echo admin_url(); ?>

With:

<?php echo base_url('admin'); ?>

Correct syntax taken from default theme.