hgweb: better English in IPv6 error message

mercurial/hgweb/server.py

@@ -275,7 +275,7 b' def create_server(ui, repo):'
 
         def __init__(self, *args, **kwargs):
             if self.address_family is None:
-                raise error.RepoError(_('IPv6 not available on this system'))
+                raise error.RepoError(_('IPv6 is not available on this system'))
             super(IPv6HTTPServer, self).__init__(*args, **kwargs)
 
     if ssl_cert: