- add .. link to package directory listing, for easy traversal

- matches style of source file directory listing

R=rsc
http://go/go-review/1024044

lib/godoc/package.html

@@ -84,6 +84,9 @@
 	<td width="25">&nbsp;</td>
 	<th align="left">Synopsis</th>
 	</tr>
+	<tr>
+	<th align="left"><a href="..">..<a></th>
+	</tr>
 	{.repeated section List}
 		<tr>
 		{Depth|padding}