Update internal/provider/resource_mackerel_channel.go
Update internal/provider/resource_mackerel_channel.go

Co-authored-by: KOJIMA Kazunori <cb738f024e110c68003096a9174a6f4b5c8031ba@gmail.com>
kjmkznr/terraform-provider-mackerel,kjmkznr/terraform-provider-mackerel