doc/frontend: fix timestamp valid value

doc/frontend.html

@@ -655,10 +655,10 @@ same behaviour. -->
  <p><tt>syntax&nbsp;</tt>&nbsp;: inline</p>
  <p><tt>valid value&nbsp;</tt>&nbsp;:</p>
   <ul>
-   <li>TAI
+   <li>tai
    	<p>The logged line will be preceded by a TAI64N timestamp 
 	(and a space) before being processed by the next action directive.</p></li>
-   <li>ISO
+   <li>iso
 	<p>The selected line will be preceded by a ISO 8601 timestamp for
 	 combined date and time representing local time according to the 
 	 systems timezone, with a space (not a 'T') between the date and 
@@ -673,11 +673,11 @@ same behaviour. -->
 
     @destination = /run/log
     @backup = 10
-    @timestamp = ISO</pre>
+    @timestamp = iso</pre>
   <pre>    [logger]
     @destination = /run/log
     @backup = 10
-    @timestamp = ISO</pre>
+    @timestamp = iso</pre>
 </ul>
 
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