Update milestone url

Github updated their interface and in the process changes the
url for the milestones.  Update the page to reference the new
address.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

index.html

@@ -163,7 +163,7 @@
 	</tr>
 	<tr>
 		<td width="25%">
-		<a href="https://github.com/zfsonlinux/zfs/issues/milestones">
+		<a href="https://github.com/zfsonlinux/zfs/milestones">
 		ZFS on Linux Roadmap </a>
 		</td>
 		<td width="25%"> </td>