expect: update website URL

expect.nist.gov no longer resolves. It seems the website has been
moved to http://expect.sourceforge.net/ , though there is also a page
on nist.gov at https://www.nist.gov/services-resources/software/expect
.

pkgs/tools/misc/expect/default.nix

···
 
   meta = with stdenv.lib; {
     description = "A tool for automating interactive applications";
-    homepage = http://expect.nist.gov/;
+    homepage = http://expect.sourceforge.net/;
     license = "Expect";
     platforms = platforms.unix;
     maintainers = with maintainers; [ wkennington ];