encoding/json: update RFC number

Existing docs mention obsolete RFC 4627. Update it with current one,
https://tools.ietf.org/html/rfc7159.

Current implementation already adhere to RFC 7159.

Fixes #22888

Change-Id: I705ec1313f6f655b3bc41d2f847b30e479bf9b15
Reviewed-on: https://go-review.googlesource.com/79995Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/json/encode.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package json implements encoding and decoding of JSON as defined in
-// RFC 4627. The mapping between JSON and Go values is described
+// Package json implements encoding and decoding of JSON text as defined in
+// RFC 7159. The mapping between JSON and Go values is described
 // in the documentation for the Marshal and Unmarshal functions.
 //
 // See "JSON and Go" for an introduction to this package: