Use macros instead of concurrency as ane example of stuff in flux

I think concurrency is not that much in flux anymore,
in large parts thanks to jnthn++

source/specification/index.html

@@ -14,7 +14,7 @@ <h2><b>Perl&nbsp;6</b> Language Design</h2>
 
     <p>
         Many parts of the language are rather mature, for example the object
-        system and large parts of the syntax. Other parts, like concurrency,
+        system and large parts of the syntax. Other parts, like macros,
         are still in flux, and wait for an implementation to try them
         out.
     </p>
@@ -44,4 +44,4 @@ <h3 class="trim-top">Notes (excerpt from <a href="http://design.perl6.org/S01.ht
       </div>
     </div>
   </div>
-</div>
+</div>