Fix vendor and purchases preferences xml reader (#626)

closes #610

Author: katzenj

lib/quickbooks/model/preferences.rb

@@ -19,7 +19,7 @@ def self.create_preference_class(*attrs, &block)
       PREFERENCE_SECTIONS = {
         :accounting_info      => %w(TrackDepartments DepartmentTerminology ClassTrackingPerTxnLine? ClassTrackingPerTxn? CustomerTerminology),
         :product_and_services => %w(ForSales? ForPurchase? QuantityWithPriceAndRate? QuantityOnHand?),
-        :vendor_and_purchase  => %w(TrackingByCustomer? BillableExpenseTracking? DefaultTerms? DefaultMarkup? POCustomField),
+        :vendor_and_purchases => %w(TrackingByCustomer? BillableExpenseTracking? DefaultTerms? DefaultMarkup? POCustomField),
         :time_tracking        => %w(UseServices? BillCustomers? ShowBillRateToAll WorkWeekStartDate MarkTimeEntiresBillable?),
         :tax                  => %w(UsingSalesTax? PartnerTaxEnabled?),
         :currency             => %w(MultiCurrencyEnabled? HomeCurrency),