fix plugin example repo link

Author: jneen

README.md

@@ -296,7 +296,7 @@ with actual code.
 
 If your language is internal or obscure, or it is taking far too long to merge
 into baseline Rouge, you can very easily write a plugin for language support.
-Check out our [plugin example repository][] for a good starting point.
+Check out our [plugin example repository][plugin-example] for a good starting point.
 
 [plugin-example]: https://github.com/rouge-ruby/rouge-plugin-example "Rouge Plugin Example"