[real-world uses] Fix misspelled url

doc/realworld.html

@@ -45,7 +45,7 @@
       <li><a href="http://antonmi.github.io/code_together/">Code together</a> (collaborative editing)</li>
       <li><a href="http://codev.it/">Codev</a> (collaborative IDE)</li>
       <li><a href="http://www.codezample.com">CodeZample</a> (code snippet sharing)</li>
-      <li><a href="http://codeio.com">Codio</a> (Web IDE)</li>
+      <li><a href="http://codio.com">Codio</a> (Web IDE)</li>
       <li><a href="http://ot.substance.io/demo/">Collaborative CodeMirror demo</a> (CodeMirror + operational transforms)</li>
       <li><a href="http://www.communitycodecamp.com/">Community Code Camp</a> (code snippet sharing)</li>
       <li><a href="http://www.ckwnc.com/">CKWNC</a> (UML editor)</li>