Fix broken links

[st0012]

What was the end-user or developer problem that led to this PR?

Not really, just spotted a broken rdoc-ref through ruby/rdoc#1241

What is your fix for the problem, implemented in this PR?

1. The rubydoc.info page doesn't exist anymore. And since ruby/ruby's documentation include rubygems's , I think we can link to its page on docs.ruby-lang.org instead.
2. The link to LICENSE.txt is broken as the file isn't synced to ruby/ruby. Since rubygems doesn't have its own API documentation atm (at least I couldn't find one), I think we can link to the source file on GitHub instead.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

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