Revert "Remove an unused prop"

Don't know what I was smoking when I removed this. Even though the
function doesn't use it, React uses the key.

This reverts commit 937b0d3.

Author: wincent

site/_core/DocsSidebar.js

@@ -12,7 +12,11 @@ import { toSlug } from './Header';
 export default ({ site, page, firstURL }) =>
   <div className="nav-docs">
     {getCategories(site, page.dir, firstURL).map(category =>
-      <SidebarForCategory pageID={page.id} category={category} />
+      <SidebarForCategory
+        pageID={page.id}
+        category={category}
+        key={category.name}
+      />
     )}
   </div>