cups-filters: align attrname with pkgname

pkgs/top-level/aliases.nix

···
   conkerorWrapper = conkeror; # added 2015-01
   cool-old-term = cool-retro-term; # added 2015-01-31
   cupsBjnp = cups-bjnp; # added 2016-01-02
+  cups_filters = cups-filters; # added 2016-08
   cv = progress; # added 2015-09-06
   debian_devscripts = debian-devscripts; # added 2016-03-23
   dwarf_fortress = dwarf-fortress; # added 2016-01-23

pkgs/top-level/all-packages.nix

···
     libusb = libusb1;
   };
 
-  cups_filters = callPackage ../misc/cups/filters.nix { };
+  cups-filters = callPackage ../misc/cups/filters.nix { };
 
   cups-pk-helper = callPackage ../misc/cups/cups-pk-helper.nix { };