org-html: fix typo in doc

Hi,

this patch fixes a typo in the documentation of org-html.

Best
Julian

>From 40483bb63d236595d6982dca26a2a3d80bfd39bc Mon Sep 17 00:00:00 2001
From: Julian Gehring <julian.gehring@googlemail.com>
Date: Wed, 29 Jun 2011 10:31:21 +0200
Subject: [PATCH] org-html: fix typo in doc

lisp/org-html.el

@@ -196,7 +196,7 @@ For example, a valid value would be:
     ]]>
    </style>
 
-If you'd like to refer to en external style file, use something like
+If you'd like to refer to an external style file, use something like
 
    <link rel=\"stylesheet\" type=\"text/css\" href=\"mystyles.css\">