content: fix broken link in blog.golang.org/json-rpc-tale-of-interfaces

The link to the source of the referred changelog was not correct
anymore. This change updates it to the github commit view.

Change-Id: Ifdd92914841672ab6062ebf1b96d8bb15ba82892
Reviewed-on: https://go-review.googlesource.com/23541
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed