cask/audit_spec: fix outdated test

We used the binary artifact as an example of an unsigned artifact
before in this test. Now that we're adding it as a signed artifact
the test has been updated with another example of an unsigned one.

Library/Homebrew/test/cask/audit_spec.rb

@@ -483,7 +483,7 @@ describe Cask::Audit, :cask do
             cask 'signing-cask-test' do
               version '1.0'
               url "https://brew.sh/index.html"
-              binary 'Audit.app'
+              artifact "example.pdf", target: "/Library/Application Support/example"
             end
           RUBY
         end