Fix bugs generating URL for back-to link

* Using wrong variable name when parsing `return_to` query parameter
  value
* Remove unused `cookieExpiration` variable

Author: Guti

docs/_layouts/page.html

@@ -49,11 +49,10 @@ <h1 class="post-title">{{ page.title | escape }}</h1>
     var returnTo = url.searchParams.get("return_to");
 
     if ( referrerUrl.hostname === registryHostname && returnTo ) {
-      var returnToUrl = new URL(returnToUrl);
+      var returnToUrl = new URL(returnTo);
 
       if ( returnToUrl.hostname === registryHostname ) {
         var now = new Date();
-        var cookieExpiration = new Date(now.getTime() + ONE_HOUR);
 
         var returnToCookie = "registry_return_to=" + returnTo + ";";
         returnToCookie += "SameSite=Strict; Secure;"