robo3t: change .desktop name

Gnome does not permit to add the application to favorites if the .desktop name is different than the binary name.
See https://unix.stackexchange.com/questions/58824/how-do-i-add-eclipse-to-my-gnome-shell-favorites.

pkgs/applications/misc/robo3t/default.nix

···
   };
 
   desktopItem = makeDesktopItem {
-    name = "Robo3T";
+    name = "robo3t";
     exec = "robo3t";
     icon = icon;
     comment = "Query GUI for mongodb";