tcp: fix tcp_rmem documentation

tcp: fix tcp_rmem documentation

commit 1d1be91254bbdd189796041561fd430f7553bb88 upstream.

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

Fixes: a337531b942b ("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: Andrea Righi <andrea.righi@canonical.com>