content: Use url.toString() instead of urlString in error message

Just so when we extract out much of this code for reuse, it won't
need a `urlString` param alongside `url`.

I think the effect is just that when the URL is a relative URL, the
error dialog will always show it as an absolute URL on the realm.

Author: chrisbobbe

lib/widgets/content.dart

@@ -1448,7 +1448,7 @@ void _launchUrl(BuildContext context, String urlString) async {
     showErrorDialog(context: context,
       title: zulipLocalizations.errorCouldNotOpenLinkTitle,
       message: [
-        zulipLocalizations.errorCouldNotOpenLink(urlString),
+        zulipLocalizations.errorCouldNotOpenLink(url.toString()),
         if (errorMessage != null) errorMessage,
       ].join("\n\n"));
   }