Fix the WebM Container Guidelines link

[alrra]

The content from http://www.webmproject.org/code/specs/container/ was moved to http://www.webmproject.org/docs/container/.

curl -sSLI http://www.webmproject.org/code/specs/container/

HTTP/1.1 404 Not Found
Content-Type: text/html; charset=UTF-8
X-Content-Type-Options: nosniff
Date: Mon, 31 Aug 2015 18:13:13 GMT
Server: sffe
Content-Length: 1531
X-XSS-Protection: 1; mode=block

[domenic]

Thanks very much! Merged as 2285630.

[annevk]

@domenic, we should add Cătălin Mariș to the acknowledgments section.