ipe: fix homepage

pkgs/applications/graphics/ipe/default.nix

···
 
   meta = {
     description = "An editor for drawing figures";
-    homepage = http://ipe7.sourceforge.net;
+    homepage = http://ipe.otfried.org;
     license = stdenv.lib.licenses.gpl3Plus;
     longDescription = ''
       Ipe is an extensible drawing editor for creating figures in PDF and Postscript format.