UPSTREAM: tcp: fix tcp_rmem documentation

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

Bug: 254441685
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>
(cherry picked from commit 1d1be912)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I2b02ff98d7a2bf715f60b3e5116a9327b06ac5a7