Fix setup instructions in readme

[mikewhitby]

Description

If you follow the README.md instructions to set up the sniffs you get an error:

[~/projects/glovre-hosting/public_html] composer config repositories:extdn-phpcs vcs git@github.com:extdn/extdn-phpcs.git

  [InvalidArgumentException]
  Setting repositories:extdn-phpcs does not exist or is not supported by this command

I've amended the command.

Sniff checklist

None relevant, very tiny PR

[schmengler]

Thanks!