Fix enum definition in TaxRate.rb

How has this ever worked this way?

Author: tvdeyen

core/app/models/spree/tax_rate.rb

@@ -14,7 +14,7 @@ class TaxRate < Spree::Base
     enum :level, {
       item: 0,
       order: 1
-    }, _suffix: true
+    }, suffix: true
 
     belongs_to :zone, class_name: "Spree::Zone", inverse_of: :tax_rates, optional: true