Update Pods.php

$single_multi does not always return "single" or "multi" as might possibly be expected, but rather a value such as "number" or "checkbox".   then the (somewhat new) logic checks for 'single' != $single_multi .    but what if $single_multi == "number" ?       
proposing to invert the logic to 'multi' == $single_multi to address pods issue 2166, this change solves that bug.