Doc: link to metadata.json is broken

[sebbASF]

The page https://forge.puppet.com/modules/puppet/postfix/readme has a link to metadata.json https://forge.puppet.com/modules/puppet/postfix/metadata.json However that just reloads the original page I think the link should be: https://github.com/voxpupuli/puppet-postfix/blob/master/metadata.json