puppetlabs-puppet_agent MSI url is wrong

PR filed from community member raising this issue

From https://github.com/puppetlabs/puppetlabs-puppet_agent/pull/178

Hey there,

At some point the url schema for Windows agents in the repo was updated to https://downloads.puppetlabs.com/windows/puppet-agent-${version}${arch}.msi, but the module is still looking for agent packages at https://downloads.puppetlabs.com/windows/puppet-agent-${arch}${version}.msi

This updates the filename to point to the correct url.

Please let me know if this needs anything else.

Thanks!