Update interfaces/dom.idl (#26988)

Source: https://github.com/w3c/webref/blob/a56faec/ed/idl/dom.idl
Build: https://travis-ci.org/w3c/webref/builds/210235866
diff --git a/interfaces/dom.idl b/interfaces/dom.idl
index 9d371fb..bd8a17a 100644
--- a/interfaces/dom.idl
+++ b/interfaces/dom.idl
@@ -43,7 +43,7 @@
 };
 
 partial interface Window {
-  [Replaceable] readonly attribute any event; // legacy
+  [Replaceable] readonly attribute (Event or undefined) event; // legacy
 };
 
 [Exposed=(Window,Worker)]