website: update vagrant PP docs for Artifactid var

website/source/docs/post-processors/vagrant.html.markdown

@@ -51,6 +51,7 @@ below, with more details about certain options in following sections.
   by this post-processor. This is a
   [configuration template](/docs/templates/configuration-templates.html).
   The variable `Provider` is replaced by the Vagrant provider the box is for.
+  The variable `ArtifactId` is replaced by the ID of the input artifact.
   By default, the value of this config is `packer_{{.Provider}}.box`.
 
 * `aws`, `virtualbox`, or `vmware` (objects) - These are used to configure