downgrade unset attr rule

Author: aireilly

.vale/styles/AsciiDoc/UnsetAttributes.yml

@@ -1,6 +1,6 @@
 ---
 extends: script
-level: error
+level: warning
 message: "Set attribute directive does not have a corresponding unset attribute directive."
 link: https://docs.asciidoctor.org/asciidoc/latest/attributes/unset-attributes/#unset-a-document-attribute-in-the-body
 scope: raw