Fix linter issues

lib/planet_express_express/constants.rb

@@ -5,7 +5,7 @@ module PlanetExpressExpress
     [:LP, :L] => 6_90,
     [:MR, :S] => 2_00,
     [:MR, :M] => 3_00,
-    [:MR, :L] => 4_00,
+    [:MR, :L] => 4_00
   }.freeze
 
   DISCOUNT_LIMIT = 10_00

lib/planet_express_express/discount_budget.rb

@@ -12,7 +12,7 @@ module PlanetExpressExpress
           if shipment[:discount] > available_budget
             shipment[:discount] = available_budget
           end
-          available_budget = available_budget - shipment[:discount]
+          available_budget -= shipment[:discount]
         end
       end
     end

lib/planet_express_express/shipment_reader.rb

@@ -1,5 +1,5 @@
 module PlanetExpressExpress
-  require 'date'
+  require "date"
   class ShipmentReader
     def initialize(prices)
       @prices = prices

spec/planet_express_express_spec.rb

@@ -16,7 +16,7 @@ RSpec.describe PlanetExpressExpress do
     PlanetExpressExpress::Formatter.new(output).format(shipments)
 
     expected_output = File.read(
-      File.expand_path("../fixtures/output.txt", __FILE__),
+      File.expand_path("../fixtures/output.txt", __FILE__)
     )
 
     output.rewind