docs: Update godoc URLs for Go packages

Since

  commit 201156ec
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Mon Dec 16 10:06:50 2019 +0000

    docs: add pages to support Go module package resolution

it's possible to use

  import "libvirt.org/libvirt-go"
  import "libvirt.org/libvirt-go-xml"

in Go programs, which will result in the Go packages being
fetched directly from libvirt.org instead of GitHub.

The godoc website offers the API documentation for the updated
imports, and that's where we should send users.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

docs/downloads.html.in

@@ -71,7 +71,7 @@
             <a href="https://github.com/libvirt/libvirt-go">github</a>
           </td>
           <td>
-            <a href="https://godoc.org/github.com/libvirt/libvirt-go">api ref</a>
+            <a href="https://godoc.org/libvirt.org/libvirt-go">api ref</a>
           </td>
         </tr>
         <tr>
@@ -206,7 +206,7 @@
             <a href="https://github.com/libvirt/libvirt-go-xml">github</a>
           </td>
           <td>
-            <a href="https://godoc.org/github.com/libvirt/libvirt-go-xml">api ref</a>
+            <a href="https://godoc.org/libvirt.org/libvirt-go-xml">api ref</a>
           </td>
         </tr>
         <tr>