encoding: fix url for fetching W3C encodings.json file

Fixes https://github.com/golang/go/issues/18862

Change-Id: I48b8ec4a8561e9590a66faa2ff0a36f8e7a01434
Reviewed-on: https://go-review.googlesource.com/36057
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
2 files changed