doc: correcting two prefix errors in idmappings.rst

Add the 'k' prefix to id 21000. And id `u1000` in the third
idmapping should be mapped to `k31000`, not `u31000`.

Signed-off-by: Aiden Ma <jiaheng.ma@foxmail.com>
Link: https://lore.kernel.org/r/tencent_4E7B1F143E8051530C21FCADF4E014DCBB06@qq.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Aiden Ma

Documentation/filesystems/idmappings.rst

@@ -63,8 +63,8 @@ what id ``k11000`` corresponds to in the second or third idmapping. The
 straightforward algorithm to use is to apply the inverse of the first idmapping,
 mapping ``k11000`` up to ``u1000``. Afterwards, we can map ``u1000`` down using
 either the second idmapping mapping or third idmapping mapping. The second
-idmapping would map ``u1000`` down to ``21000``. The third idmapping would map
-``u1000`` down to ``u31000``.
+idmapping would map ``u1000`` down to ``k21000``. The third idmapping would map
+``u1000`` down to ``k31000``.
 
 If we were given the same task for the following three idmappings::