Partial spec of error message for readability

This avoids Rubocop warnings. We don't need to test for the whole
message. The tested part is unique already and quicker to read.

spec/system/admin/order_spec.rb

@@ -1104,12 +1104,12 @@ def serching_for_customers
 
             # the New invoice button + the warning should be visible
             expect(page).to have_link "Create or Update Invoice"
-            expect(page).to have_content "The order has changed since the last invoice update. The invoice shown here might not be up-to-date anymore."
+            expect(page).to have_content "The order has changed since the last invoice update."
             click_link "Create or Update Invoice"
 
             # and disappear after clicking
             expect(page).to have_no_link "Create or Update Invoice"
-            expect(page).to_not have_content "The order has changed since the last invoice update. The invoice shown here might not be up-to-date anymore."
+            expect(page).to_not have_content "The order has changed since the last invoice update."
 
             # creating an invoice, displays a second row
             expect(page.find("table").text).to have_content(table_contents)