Update Description of Module to fix 404 link

Description of this module at https://www.drupal.org/project/commerce_authnet →

Under the heading
Registering with Authorize.Net

There is a link text "register a test account" linking to https://developer.authorize.net/testaccount.html . That is a 404 page.

Your README correctly links to the new page for that:
https://developer.authorize.net/hello_world/sandbox.html

I just happened to be following the module page's piece vs the readme. Worth a quick update to whoever can make that edit.