cmd/godoc: fix BUGS heading in text mode

All the headings are CAPS not Title.

Fixes #4843.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7792043

lib/godoc/package.txt

@@ -65,7 +65,7 @@ TYPES
 
 */}}{{with $.Notes}}
 {{range $marker, $content := .}}
-{{noteTitle $marker}}s
+{{$marker}}S
 
 {{range $content}}{{comment_text . "    " "\t"}}
 {{end}}{{end}}{{end}}{{end}}{{/*