tree-wide: s/GTK+/GTK/g

GTK was renamed.

nixos/doc/manual/configuration/customizing-packages.xml

@@ -24,8 +24,8 @@
  <para>
   Apart from high-level options, it’s possible to tweak a package in almost
   arbitrary ways, such as changing or disabling dependencies of a package. For
-  instance, the Emacs package in Nixpkgs by default has a dependency on GTK+ 2.
-  If you want to build it against GTK+ 3, you can specify that as follows:
+  instance, the Emacs package in Nixpkgs by default has a dependency on GTK 2.
+  If you want to build it against GTK 3, you can specify that as follows:
 <programlisting>
 <xref linkend="opt-environment.systemPackages"/> = [ (pkgs.emacs.override { gtk = pkgs.gtk3; }) ];
 </programlisting>
@@ -33,7 +33,7 @@
   function that produces Emacs, with the original arguments amended by the set
   of arguments specified by you. So here the function argument
   <varname>gtk</varname> gets the value <literal>pkgs.gtk3</literal>, causing
-  Emacs to depend on GTK+ 3. (The parentheses are necessary because in Nix,
+  Emacs to depend on GTK 3. (The parentheses are necessary because in Nix,
   function application binds more weakly than list construction, so without
   them, <xref linkend="opt-environment.systemPackages"/> would be a list with
   two elements.)