Fix tests for deposit derived var

app/models/form/sales/questions/deposit_amount.rb

@@ -9,5 +9,6 @@ class Form::Sales::Questions::DepositAmount < ::Form::Question
     @width = 5
     @prefix = "£"
     @hint_text = "Enter the total cash sum paid by the buyer towards the property that was not funded by the mortgage"
+    @derived = true
   end
 end

spec/models/form/sales/questions/deposit_amount_spec.rb

@@ -27,8 +27,8 @@ RSpec.describe Form::Sales::Questions::DepositAmount, type: :model do
     expect(question.type).to eq("numeric")
   end
 
-  it "is not marked as derived" do
-    expect(question.derived?).to be false
+  it "is marked as derived" do
+    expect(question.derived?).to be true
   end
 
   it "has the correct hint" do