doc: idmappings: fix an error and rephrase a paragraph

If the modified paragraph is referring to the idmapping mentioned in the
previous paragraph (i.e. `u0:k10000:r10000`), then it is `u0` that the
upper idmapset starts with, not `u1000`.

Fix this error and rephrase this paragraph a bit to make this reference
more explicit.

Reported-by: Wang Lei <wanglei249@huawei.com>
Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
Message-Id: <20230816033210.914262-1-gongruiqi@huaweicloud.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>