use .many? instead of `.count > 1` for collection

super minor, per https://github.com/refinery/refinerycms/commit/02c74dad15a7af380037e29820e27e208dccae1f#commitcomment-2450022
(so as to be consistent. `ack \.many\?` shows plenty)
Merge pull request #2153 from harleyttd/patch-011812

use ".many?" instead of ".count > 1" for collection