[vhdl mode] Fix a typo

mode/vhdl/vhdl.js

 // CodeMirror, copyright (c) by Marijn Haverbeke and others
 // Distributed under an MIT license: http://codemirror.net/LICENSE
 
-// Originall written by Alf Nielsen, re-written by Michael Zhou
+// Originally written by Alf Nielsen, re-written by Michael Zhou
 (function(mod) {
   if (typeof exports == "object" && typeof module == "object") // CommonJS
     mod(require("../../lib/codemirror"));