[css-values] fix markup error

Author: tabatkins

css-values/Overview.bs

@@ -1174,7 +1174,7 @@ Frequency Units: the <<frequency>> type and ''Hz'', ''kHz'' units</h3>
 	<dl export dfn-type=value dfn-for="<frequency>">
 	  <dt><dfn id="Hz">Hz</dfn>
 	  <dd>Hertz. It represents the number of occurrences per second.
-	  <dt><dfn id="kHz>kHz</dfn>
+	  <dt><dfn id="kHz">kHz</dfn>
 	  <dd>KiloHertz. A kiloHertz is 1000 Hertz.
 	</dl>