Fix buglet in readable theme's post template.

Thanks to Matthias Wimmer for the report and fix.

themes/readable/post.tmpl

   <p>
     {if $post.tags}
       Tagged as {foreach $tag in $post.tags}
-        <a href="{$config.domain}/tag/{$tag.slug}{$config.pageExt}">{$tag.name}</a>{nil}
+        <a href="{$config.domain}/tag/{$tag.slug}.{$config.pageExt}">{$tag.name}</a>{nil}
           {if not isLast($tag)},{sp}{/if}
       {/foreach}
     {/if}