doc: correcting the idmapping mount example

In step 2, we obtain the kernel id `k1000`. So in next step (step
3), we should translate the `k1000` not `k21000`.

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Link: https://lore.kernel.org/r/20240816063611.1961910-1-lihongbo22@huawei.com


Signed-off-by: Christian Brauner <brauner@kernel.org>