Fix typo: a -> an

'There is **an** odd thing...'

Author: jasonflorentino

chapter7_evaluation.html

@@ -57,7 +57,7 @@ <h2 id='trees'>Trees</h2> <hr/>
 
 <h2 id='recursion'>Recursion</h2> <hr/>
 
-<p>There is a odd thing about this tree structure. It refers to itself. Each of its children are themselves trees again, and the children of those children are trees yet again. Just like our languages and re-write rules, data in this structure contains repeated substructures that resemble their parents.</p>
+<p>There is an odd thing about this tree structure. It refers to itself. Each of its children are themselves trees again, and the children of those children are trees yet again. Just like our languages and re-write rules, data in this structure contains repeated substructures that resemble their parents.</p>
 
 <div class='pull-right alert alert-warning' style="margin: 15px; text-align: center;">
   <img src="/static/img/recursion.png" alt="recursion" class="img-responsive" width="302px" height="371px"/>