go.tools/godoc: fix search heading anchors

Right now they're all "Global" so you can't click use the table of
contents headings for "Types", "Functions", and "Methods".

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/118100043

godoc/static/searchdoc.html

@@ -5,7 +5,7 @@
 -->
 {{range $key, $val := .Idents}}
         {{if $val}}
-                <h2 id="Global">{{$key.Name}}</h2>
+		<h2 id="{{$key.Name}}">{{$key.Name}}</h2>
                 {{range $val}}
 	                {{$pkg_html := pkgLink .Path | html}}
 			{{if eq "Packages" $key.Name}}

godoc/static/static.go

@@ -2601,7 +2601,7 @@ function PlaygroundOutput(el) {
 -->
 {{range $key, $val := .Idents}}
         {{if $val}}
-                <h2 id="Global">{{$key.Name}}</h2>
+		<h2 id="{{$key.Name}}">{{$key.Name}}</h2>
                 {{range $val}}
 	                {{$pkg_html := pkgLink .Path | html}}
 			{{if eq "Packages" $key.Name}}