Fix reference to LICENSE file (#488)

Author: AyanSinhaMahapatra

source-map.js

@@ -1,6 +1,6 @@
 /*
  * Copyright 2009-2011 Mozilla Foundation and contributors
- * Licensed under the New BSD license. See LICENSE.txt or:
+ * Licensed under the New BSD license. See LICENSE or:
  * http://opensource.org/licenses/BSD-3-Clause
  */
 exports.SourceMapGenerator =