tcp: fix tcp_rmem documentation

stable inclusion
from stable-5.10.21
commit 9adbc25b0e309500f8fb139adfb397b762952230
bugzilla: 50609

--------------------------------

commit 1d1be912 upstream.

tcp_rmem[1] has been changed to 131072, we should update the documentation
to reflect this.

Fixes: a337531b ("tcp: up initial rmem to 128KB and SYN rwin to around 64KB")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Zhibin Liu <zhibinliu@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>