rsstail: update repository location

Project's repository has been moved. Former location,
`github.com/flok99/rsstail`, is no longer accessible. The author moved
the code repository to `github.com/folkertvanheusden/rsstail`.

I've verified this in the following ways.

First of all, by checking project's homepage, i.e.
https://www.vanheusden.com/rsstail/. It indeed points to the new
location.

Second of all, I've emailed the author, Folkert van Heusden, and they
confirmed the new location.

What is more, one can also note that the commit hash is still the same.

pkgs/applications/networking/feedreaders/rsstail/default.nix

···
     sha256 = "12p69i3g1fwlw0bds9jqsdmzkid3k5a41w31d227i7vm12wcvjf6";
     rev = "6f2436185372b3f945a4989406c4b6a934fe8a95";
     repo = "rsstail";
-    owner = "flok99";
+    owner = "folkertvanheusden";
   };
 
   buildInputs = [ libmrss ] ++ lib.optionals stdenv.isDarwin [ libiconv ];