Add value for simd feature test macro

Author: jaredhoberock

general.html

@@ -78,7 +78,7 @@ <h1>Feature-testing recommendations</h1>
           <td><ins>Data-Parallel Vector Types & Operations</ins></td>
           <td><ins><cxx-ref to="parallel.simd"</cxx-ref></ins></td>
           <td><ins><code>__cpp_lib_experimental_parallel_simd</code></ins>
-          <td><ins>????</td>
+          <td><ins>201803</ins></td>
           <td><ins><code>&lt;experimental/simd&gt;</code></ins></td>
         </tr>
       </thead>