acme/autocert: clarify that multiple names are allowed

Change-Id: Ib5111388859b36c1989aad1a1948bd83ab01b7e0
Reviewed-on: https://go-review.googlesource.com/132328
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

acme/autocert/example_test.go

@@ -24,7 +24,7 @@ func ExampleManager() {
 	m := &autocert.Manager{
 		Cache:      autocert.DirCache("secret-dir"),
 		Prompt:     autocert.AcceptTOS,
-		HostPolicy: autocert.HostWhitelist("example.org"),
+		HostPolicy: autocert.HostWhitelist("example.org", "www.example.org"),
 	}
 	s := &http.Server{
 		Addr:      ":https",