godoc: <pre> must not occur inside <p>

Fixes #2532

R=golang-dev, dr.volker.dobler, rsc
CC=golang-dev
https://golang.org/cl/5450115

lib/godoc/example.html

@@ -5,10 +5,10 @@
 	<div class="expanded">
 		<p class="exampleHeading">▾ Example</p>
 		<p>Code:</p>
-		<p class="code"><pre>{{.Code}}</pre></p>
+		<pre class="code">{{.Code}}</pre>
 		{{if .Output}}
 		<p>Output:</p>
-		<p class="output"><pre>{{html .Output}}</pre></p>
+		<pre class="output">{{html .Output}}</pre>
 		{{end}}
 	</div>
 </div>

lib/godoc/package.html

@@ -51,8 +51,8 @@
 	{{comment_html .Doc}}
 	{{if $.IsPkg}}
 		{{with .Filenames}}
-			<p>
 			<h4>Package files</h4>
+			<p>
 			<span style="font-size:90%">
 			{{range .}}
 				<a href="/{{.|srcLink}}">{{.|filename|html}}</a>
@@ -88,7 +88,7 @@
 		{{$tname_html := node_html .Type.Name $.FSet}}
 		<h2 id="{{$tname_html}}">type <a href="/{{posLink_url .Decl $.FSet}}">{{$tname_html}}</a></h2>
 		{{comment_html .Doc}}
-		<p><pre>{{node_html .Decl $.FSet}}</pre></p>
+		<pre>{{node_html .Decl $.FSet}}</pre>
 		{{range .Consts}}
 			{{comment_html .Doc}}
 			<pre>{{node_html .Decl $.FSet}}</pre>