joesimms: fixed link to tree traversal information in docs

manual/docs/Hierarchical data - Introduction - Traversing or Walking Trees.php

-<p>You can traverse a Tree in different ways, please see here for more information http://en.wikipedia.org/wiki/Tree_traversal.</p>
+<p>You can traverse a Tree in different ways, please see here for more information <a href="http://en.wikipedia.org/wiki/Tree_traversal" >http://en.wikipedia.org/wiki/Tree_traversal</a>.</p>
 
 <p>The most common way of traversing a tree is Pre Order Traversal as explained in the link above, this is also what is known as walking the tree, this is the default approach when traversing a tree in Doctrine, however Doctrine does plan to provide support for Post and Level Order Traversal (not currently implemented)</p>
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