[www] cxx_status: Update Reflection TS to Cologne draft

Summary:
As of the 2019 Cologne meeting, according to its minutes (N4826), N4818
is the draft of the Reflection TS.

Reviewers: rsmith, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75524

Author: hubert-reinterpretcast

clang/www/cxx_status.html

@@ -1321,7 +1321,7 @@ <h2 id="ts">Technical specifications and standing documents</h2>
     </tr>
     <tr>
       <td>[DRAFT TS] Reflection</td>
-      <td><a href="https://wg21.link/n4746">N4746</a></td>
+      <td><a href="https://wg21.link/n4818">N4818</a></td>
       <td></td>
       <td class="none" align="center">No</td>
     </tr>