Mark base*_decode_update change as done.

misc/plan.html

@@ -60,7 +60,7 @@
     sizes.</span> <span class='postponed'>(And possibly also for
     cast128).</span>
   </p>
-  <p class='should'>
+  <p class='done'>
     Don't require initialization of *dst_length for base*_decode_update.
   </p>
   <p class='should'>