fix url schema for webrdp example

[stirby]

Tested the corrected URL on dogfood.

Was

module "windows_rdp" {
  source      = "registry.coder.com/coder/module/windows-rdp"
  ...
}

Now

module "windows_rdp" {
  source      = "registry.coder.com/modules/windows-rdp/coder"
  ...
}

[matifali]

It was an overlook from last review.
Thank you for noting this.