fix build error: assignment mismatch

These errors appear when I build this package: ``` # 41.neocities.org/google/internal/play internal/play/play.go:67:25: assignment mismatch: 2 variables but apk.Url returns 1 value internal/play/play.go:68:22: assignment mismatch: 2 variables but apk.Field1 returns 1 value internal/play/play.go:82:25: assignment mismatch: 2 variables but obb.Url returns 1 value internal/play/play.go:83:22: assignment mismatch: 2 variables but obb.Field1 returns 1 value internal/play/play.go:91:16: assignment mismatch: 2 variables but deliver.Url returns 1 value ``` The errors were introduced in a5493ee recently. This PR fixes the errors.