Fix a minor typo (#683)

I found this small typo in the documentation. I'm unsure if this is the right way to contribute. Hopefully this helps :)

Author: Talkashie

help/C/xreader.html

@@ -225,7 +225,7 @@ <h2 id="to-open-a-document">To Open A Document</h2>
 <li><p>Click Open. xreader displays the name of the document in the titlebar of the window.</p></li>
 </ol>
 <p>To open another document, choose FileOpen again. xreader opens each file in a new window.</p>
-<p>If you try to open a document with format that xreader does not recognize, the application displays an error message.</p>
+<p>If you try to open a document with a format that xreader does not recognize, the application displays an error message.</p>
 <h2 id="to-navigate-through-a-document">To Navigate Through a Document</h2>
 <p>You can navigate through a file as follows:</p>
 <ul>