docs(marketing): add parameter placeholder to /plannotator-review command

For provenance purposes, this commit was AI assisted.

Author: backnotprop

apps/marketing/src/pages/index.astro

@@ -323,7 +323,7 @@ import AnnoReplace from '../components/landing/AnnoReplace.astro';
               <span class="cap-desc">Annotate the agent's last message</span>
             </div>
             <div class="cap-item">
-              <code class="cap-cmd">/plannotator-review</code>
+              <code class="cap-cmd">/plannotator-review &lt;null|pr-url&gt;</code>
               <span class="cap-desc">Code review for local changes or GitHub and GitLab PRs</span>
             </div>
           </div>