godoc: don't display navigation list with only 1 element

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/4547056

doc/godocs.js

@@ -104,7 +104,7 @@ function godocs_generateTOC() {
     }
   }
 
-  if (!toc_items.length) { return; }
+  if (toc_items.length <= 1) { return; }
 
   var dl1 = document.createElement('dl');
   var dl2 = document.createElement('dl');