Correct Ghostscript 9.2.1 to 9.21 as per advisory

Author: wvu

modules/exploits/unix/fileformat/ghostscript_type_confusion.rb

@@ -15,7 +15,7 @@ def initialize(info = {})
       'Description'     => %q{
         This module exploits a type confusion vulnerability in Ghostscript that can
         be exploited to obtain arbitrary command execution. This vulnerability affects
-        Ghostscript version 9.2.1 and earlier and can be exploited through libraries
+        Ghostscript version 9.21 and earlier and can be exploited through libraries
         such as ImageMagick and Pillow.
       },
       'Author'          => [