field code should be matched as 'begin'

Author: gg582

src/languages/freedesktop.js

@@ -8,7 +8,7 @@ Website: https://www.freedesktop.org/
 export default function(hljs) {
   const FIELD_CODES = {
     className: 'variable',
-    match: /%[a-zA-Z]/,
+    begin: /%[a-zA-Z]/,
     relevance: 0
   };