Exclude from nav.

Change-Id: Ibe001e747ef64c5c232fdaeb8b47167490a85154
diff --git a/404.md b/404.md
index 129881e..668dd59 100644
--- a/404.md
+++ b/404.md
@@ -1,6 +1,7 @@
 ---
 layout: page
 title: Not Found
+exclude: true
 permalink: /404.html
 ---