switch gemspec to be dual

Author: thompson-tomo

LICENSE.rdoc

@@ -1,10 +1,11 @@
-# SPDX-License-Identifier: Ruby
+# SPDX-License-Identifier: Ruby or GPL-2
 
 = License
 
 RDoc is copyrighted free software.
 
-You can redistribute it and/or modify it under the terms of the Ruby License.
+You can redistribute it and/or modify it under either the terms of the GPL
+version 2 (see the file GPL), or the conditions below:
 
 1. You may make and give away verbatim copies of the source form of the
    software without restriction, provided that you duplicate all of the

lib/rdoc/rd.rb

@@ -14,7 +14,7 @@
 # You can find the original source for rdtool at
 # https://github.com/uwabami/rdtool/
 #
-# You can use, re-distribute or change these files under Ruby's License.
+# You can use, re-distribute or change these files under Ruby's License or GPL.
 #
 # 1. You may make and give away verbatim copies of the source form of the
 #    software without restriction, provided that you duplicate all of the

rdoc.gemspec

@@ -26,7 +26,7 @@ RDoc produces HTML and command-line documentation for Ruby projects.
 RDoc includes the +rdoc+ and +ri+ tools for generating and displaying documentation from the command-line.
   DESCRIPTION
   s.homepage = "https://ruby.github.io/rdoc"
-  s.licenses = ["Ruby"]
+  s.licenses = ["Ruby", "GPL"]
 
   s.metadata["homepage_uri"] = s.homepage
   s.metadata["source_code_uri"] = "https://github.com/ruby/rdoc"