Do not link to ruby-doc.org

ruby-doc.org is a commercial site that serves ads, we have an official
Ruby documentation hosting site at docs.ruby-lang.org.

It's not well ranked in search engines but it's the only official host
of Ruby documentation and it can't be hijacked by random folks to serve
ads or potentially much more malicious content to unsuspecting folks
looking at the Try Ruby playground.

Author: olivierlacan

source/playground.html.markdown

@@ -9,14 +9,14 @@ description: Play around with Ruby programs
     <div id="tryruby-content">
       <div class="playground-iframe-wrapper">
         <iframe
-          src="https://www.ruby-doc.org/core/Kernel.html"
-        >www.ruby-doc.org</iframe>
+          src="https://docs.ruby-lang.org/en/master/"
+        >Ruby Documentation</iframe>
       </div>
 
       <p>
         In the Playground you can try any Ruby code you like.<br />
         The
-        <a href="https://www.ruby-doc.org/core/Kernel.html" target="_blank">Ruby documentation</a>
+        <a href="https://docs.ruby-lang.org/en/master/" target="_blank">Official Ruby documentation</a>
         is included.
       </p>
     </div>