Update documentation for SelectQuery::select()

When using a Closure as first parameter of SelectQuery::select(), it
will receive the SelectQuery instance as parameter.
However, SelectQuery::count() is not an existing method and
FunctionsBuilder returned by SelectQuery::func() must be used instead.

src/Database/Query/SelectQuery.php

@@ -146,7 +146,7 @@ public function all(): iterable
      * $query->select('id', true); // Resets the list: SELECT id
      * $query->select(['total' => $countQuery]); // SELECT id, (SELECT ...) AS total
      * $query->select(function ($query) {
-     *     return ['article_id', 'total' => $query->count('*')];
+     *     return ['article_id', 'total' => $query->func()->count('*')];
      * })
      * ```
      *