acme: specify which version exactly is implemented

The WG has recently published v3 of the spec.
The package implementation is still at v2. So is Let's Encrypt, to be
honest.

Regardless, I think it makes sense to point to the exact version
the package is compatible with.

Change-Id: Ibb3fc51aae363d72f67f7e876ef46c4f51c8da0b
Reviewed-on: https://go-review.googlesource.com/25451
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

acme/internal/acme/acme.go

@@ -2,8 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package acme provides an ACME client implementation.
-// See https://ietf-wg-acme.github.io/acme/ for details.
+// Package acme provides an implementation of the
+// Automatic Certificate Management Environment (ACME) spec.
+// See https://tools.ietf.org/html/draft-ietf-acme-acme-02 for details.
 //
 // This package is a work in progress and makes no API stability promises.
 package acme