Revise Intro for file directory tree with computed properties

Fix editorial issues for issue #223.

modified examples/treeview/treeview-1/treeview-1a.html:
* Fix typos and spelling errors in introductory paragraphs.
* Added brief explanation of the function of  the example to introduction.
* Minor editorial revisions to the structure and wording of the introduction.

examples/treeview/treeview-1/treeview-1a.html

@@ -25,7 +25,16 @@ <h1>File Directory Treeview Example Using Computed Properties</h1>
         Feedback and progress on editorial work on this page are being tracked in 
         <a href="https://github.com/w3c/aria-practices/issues/223">issue 223.</a>
       </p>
-      <p>This example implements the features of the <a href="../../../#TreeView">Treeview Design Pattern</a> for simulation of a file selector.  This example relies on the browser to compute values for <code>aria-setsize</code>, <code>aria-posinset</code> and <code>arial-level</code>.   The ARIA 1.0 Spectification for these properties states the browser can compute these values, but is not required to.</p>
+      <p>
+        The below example implements the 
+        <a href="../../../#TreeView">Treeview Design Pattern</a> 
+        to simulate a file selector.  
+        When users activate an item that represents a file name in the below tree, the name of the selected file appears in the read-only edit field next to the tree.
+      </p>
+      <p>
+        This example relies on the browser to compute values for <code>aria-setsize</code>, <code>aria-posinset</code> and <code>aria-level</code>.   
+        The ARIA 1.0 specification for these properties states that browsers can, but are not required to, compute these values.
+       </p>
       <p>Similar examples include: </p>
       <ul>
         <li><a href="../treeview-1/treeview-1b.html">File Directory Treeview using <em>declared</em> properties</a></li>