set ?go-get=1 page Content-Type to "text/plain"

There's no HTML on them anyway, so no point in using "text/html".
This way, it's easier to see (or show) their content in a browser,
no need to use view-source.

code.go

@@ -71,7 +71,7 @@ func (h *codeHandler) ServeCodeMaybe(w http.ResponseWriter, req *http.Request) (
 		// Handle ?go-get=1 requests, serve a go-import meta tag page.
 		if req.Method == http.MethodGet && req.URL.Query().Get("go-get") == "1" {
 			metrics.IncGoGetRequestsTotal(d.ImportPath)
-			w.Header().Set("Content-Type", "text/html; charset=utf-8")
+			w.Header().Set("Content-Type", "text/plain; charset=utf-8")
 			fmt.Fprintf(w, `<meta name="go-import" content="%[1]s git https://%[1]s">
 <meta name="go-import" content="%[1]s mod https://dmitri.shuralyov.com/api/module">
 <meta name="go-source" content="%[1]s https://%[1]s https://gotools.org/%[2]s https://gotools.org/%[2]s#{file}-L{line}">`, d.RepoRoot, d.ImportPath)