remove "em" unit from td width specification

R=rsc
http://go/go-review/1018037

lib/godoc/dirs.html

 <table class="layout">
 <tr><td colspan="2"><a href="{Path|path}">{Name|html}</a></td></tr>
 {.repeated section Dirs}
-	<tr><td width="25em"></td><td>{@|dir}</td></tr>
+	<tr><td width="25"></td><td>{@|dir}</td></tr>
 {.end}
 </table>