Fix Emacs modeline in DTrace sample.

Apparently, the DTrace mode for Emacs is called dtrace-script:
https://github.com/dotemacs/dtrace-script-mode

lib/linguist/languages.yml

@@ -711,6 +711,8 @@ DM:
 
 DTrace:
   type: programming
+  aliases:
+  - dtrace-script
   extensions:
   - .d
   interpreters:

samples/DTrace/javascript-trace.d

@@ -1,4 +1,4 @@
-/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* -*- Mode: dtrace-script; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* ***** BEGIN LICENSE BLOCK *****
  * Version: MPL 1.1/GPL 2.0/LGPL 2.1
  *