fix link to the portworx README

Markdown links don't generally require the `.md` extension when viewed in generated documentation, but in generated GitHub links the `.md` extension is needed. 

There are conflicting interests in how to represent these links. What works on GitHub won't work in generated docs and neither of those approaches work on the Terraform Registry: 
* modules/portworx/README
* modules/portworx/README.md
* https://github.com/equinix/terraform-metal-anthos-on-baremetal/blob/master/modules/portworx/README.md
* https://registry.terraform.io/modules/equinix/anthos-on-baremetal/metal/latest/submodules/portworx.

This PR includes two links, the former should keep GitHub users on GitHub, the latter will work on the Terraform Registry.

README.md

@@ -247,4 +247,4 @@ To enable a storage module, set the `storage_module` variable to the name of the
 
   When enabled, Portworx will manage the local disks attached to each worker node, providing a fault tolerant distributed storage solution.
 
-  [Read more about the Portworx module](modules/portworx/README).
+  [Read more about the Portworx module](modules/portworx/README.md) ([also available on the Terraform Registry](https://registry.terraform.io/modules/equinix/anthos-on-baremetal/metal/latest/submodules/portworx)).