Fix missing final newline from PR 158

Author: dtolnay

_src/variant-attrs.md

@@ -105,4 +105,4 @@
   and deserialize this variant as untagged, i.e. simply as the variant's data
   with no record of the variant name.
 
-  Untagged variants must be ordered last in the enum definition.
+  Untagged variants must be ordered last in the enum definition.