Fixed typo

core/css/styles.css

@@ -245,7 +245,7 @@ fieldset.warning a { color:#b94a48 !important; font-weight:bold; }
 	position:fixed; float:left; width:64px; padding-top:3.5em; z-index:75; height:100%;
 	background:#383c43 url('../img/noise.png') repeat; border-right:1px #333 solid;
 	-moz-box-shadow:0 0 7px #000; -webkit-box-shadow:0 0 7px #000; box-shadow:0 0 7px #000;
-	overflow:hidden; box-sizing:border-box; -moz-box-sizing:birder-box;
+	overflow:hidden; box-sizing:border-box; -moz-box-sizing:border-box;
 }
 #navigation:hover { overflow-y:auto; }
 #navigation a {