Fix xml mode

Change-Id: Ib419a2fadea2eb321559269b8e23ed14c0c20f3b
(cherry picked from commit 06719abee6c38d17008599f074050db5153bffa3)

web/element/language.ts

diff --git a/web/element/language.ts b/web/element/language.ts
index 41b1d81..5f380d3 100644
--- a/web/element/language.ts
+++ b/web/element/language.ts
@@ -171,7 +171,7 @@
     case 'text/x-crystal':
       return StreamLanguage.define(crystal);
     case 'application/xml':
-      return xml;
+      return xml();
     case 'text/css':
       return css();
     case 'text/x-less':