[DOC] Update the license of Darkfish (#922)

Fix #921.

Author: nobu

LEGAL.rdoc

@@ -5,7 +5,7 @@
 The files in this distribution are covered by the Ruby license (see LICENSE) except the features mentioned below:
 
 Darkfish::
-  Darkfish was written by Michael Granger and is included under the MIT
+  Darkfish was written by Michael Granger and is included under the BSD 3-Clause
   license.  Darkfish contains images from the Silk Icons set by Mark James.
 
   See lib/rdoc/generator/darkfish.rb for license information.