Fix asciidoc incorrect link syntax

CONTRIBUTING.adoc

@@ -20,7 +20,7 @@ added after the original pull request but before a merge.
   the ``Importing into eclipse'' instructions below you should get project specific
   formatting automatically. You can also import formatter settings using the
   `eclipse-code-formatter.xml` file from the `eclipse` folder. If using IntelliJ, you can
-  use the [Eclipse Code Formatter Plugin](http://plugins.jetbrains.com/plugin/6546)
+  use the http://plugins.jetbrains.com/plugin/6546[Eclipse Code Formatter Plugin]
   to import the same file.
 * Make sure all new `.java` files to have a simple Javadoc class comment with at least an
   `@author` tag identifying you, and preferably at least a paragraph on what the class is