chore: Have license content match gemspec (#1654)

This updates the gemspec to explicitly mention the GPL license as per
the license files which also make clear the license expression.

This ensures that tools such as Fossa are not generating false positives
for using just GPL licenses in Apache licensed projects.

---------

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

Author: thompson-tomo

LICENSE.rdoc

@@ -1,3 +1,7 @@
+:stopdoc:
+SPDX-License-Identifier: Ruby or GPL-2.0-only
+:startdoc:
+
 = License
 
 RDoc is copyrighted free software.

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-2.0-only"]
 
   s.metadata["homepage_uri"] = s.homepage
   s.metadata["source_code_uri"] = "https://github.com/ruby/rdoc"