html/charset/testdata: update licensing info in README
html/charset/testdata: update licensing info in README

Update the licensing info following the instructions at
http://www.w3.org/Consortium/Legal/2008/04-testsuite-copyright.html#howtouse

Fixes golang/go#10398.

Change-Id: Ib37b2a696a5287f41d4e85da4eb7ec1cbb346301
Reviewed-on: https://go-review.googlesource.com/8978
Reviewed-by: Martin Packman <0bb6cef66e112abc53be25953b15a8514717f831@canonical.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

zabawaba99/net,veliamsli/net,golang/net,mckinney-subgraph/net,w4ngyi/net,veliamsli/net,cmars/tools,cesanta/goxnet,flowlo/go-net,cmars/tools,muzining/net,raboof/net,raboof/net,yangxianjun/net,w4ngyi/net,DramaFever/golang.org-x-net,mckinney-subgraph/net,dafyddcrosby/net,albertjin/golang-net,muzining/net,flowlo/go-net,kiip/net,kiip/net,albertjin/golang-net,dafyddcrosby/net,gomini/net,zabawaba99/net,subuk/net,cesanta/goxnet,cmars/tools,DramaFever/golang.org-x-net,cmars/tools,yangxianjun/net,gomini/net,subuk/net