Fix extension registry link in README.md

[PixievoltNo1]

https://brackets-registry.aboutweb.com/ is unreachable due to an HTTPS error, so let's switch that to the brackets.io equivalent.

This wiki page also has a link to the aboutweb.com site that needs updating: https://github.com/adobe/brackets/wiki/Brackets-Extensions

[petetnt]

Hi @PikadudeNo1!

Thanks for this contribution! 👍 Before we can merge it, you'll need to sign our Contributor License Agreement (CLA) at http://dev.brackets.io/brackets-contributor-license-agreement.html.

[PixievoltNo1]

Done. I'd have felt better about it if that page used HTTPS.

[petetnt]

Thanks @PikadudeNo1! I didn't realize that URL was still only available only thought http, but I did the some internal actions to move all traffic on brackets related URLS behind https.