fix Open Graph og:image link (#6612)

Author: ashimokawa

templates/base/head.tmpl

@@ -144,7 +144,7 @@
 {{else}}
 	<meta property="og:title" content="{{AppName}}">
 	<meta property="og:type" content="website" />
-	<meta property="og:image" content="{{AppSubUrl}}/img/gitea-lg.png" />
+	<meta property="og:image" content="{{AppUrl}}/img/gitea-lg.png" />
 	<meta property="og:url" content="{{AppUrl}}" />
 	<meta property="og:description" content="{{MetaDescription}}">
 {{end}}