README: GTK+ is a acronym

Signed-off-by: Diego Viola <diego.viola@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>

README

@@ -161,7 +161,7 @@ CONFIGURING the kernel:
 
      "make xconfig"     X windows (Qt) based configuration tool.
 
-     "make gconfig"     X windows (Gtk) based configuration tool.
+     "make gconfig"     X windows (GTK+) based configuration tool.
 
      "make oldconfig"   Default all questions based on the contents of
                         your existing ./.config file and asking about