Fix rdoc-related warning when running doc rake task

I was seeing the following warning:

    warning: rdoc was loaded from the standard library,
    but will no longer be part of the default gems
    starting from Ruby 3.5.0.

Gemfile

@@ -9,6 +9,7 @@ group :development do
   gem 'rake'
 
   if ENV['MOCHA_GENERATE_DOCS']
+    gem 'rdoc'
     gem 'redcarpet'
     gem 'yard'
   end