Update param name of site description

The internal templates expect the site description to be at
Site.Params.Description, so I've updated to use that.

layouts/partials/head.html

@@ -2,7 +2,7 @@
 	<meta charset="UTF-8" />
 	<meta http-equiv="content-language" content="en-us" />
 	<meta name="author" content="{{ .Site.Params.Author }}" />
-	<meta name="description" content="{{ if .Description }}{{ .Description }}{{ else }}{{ .Site.Params.SiteDescription }}{{ end }}" />
+	<meta name="description" content="{{ if .Description }}{{ .Description }}{{ else }}{{ .Site.Params.Description }}{{ end }}" />
 	{{- $keywords := "" }}
 	{{- if .Params.Tags }}{{ $keywords = delimit .Params.Tags "," }}{{ end }}
 	{{- if .IsHome }}{{ $keywords = delimit .Site.Params.Tags "," }}{{ end }}