template: remove duplicate element id

Fixes golang/go#14435

Change-Id: Icf318cc2a8c71258e1c8f415e588e508c29f3a06
Reviewed-on: https://go-review.googlesource.com/19782
Reviewed-by: Rob Pike <r@golang.org>

template/index.tmpl

@@ -24,7 +24,7 @@
 			{{/* The dashes in the following template directives
 			     ensure the generated HTML of this list contains no
 			     extraneous spaces or line breaks. */}}
-			<ul id="sidebar">
+			<ul>
 				{{- range .Links}}
 				<li><a href="{{.URL}}">{{.Title}}</a></li>
 				{{- end}}