doc: Change the min default value of tcp_wmem/tcp_rmem.

The SK_MEM_QUANTUM was changed from PAGE_SIZE to 4096. And the
tcp_wmem/tcp_rmem min default values are 4096.

Fixes: bd68a2a8 ("net: set SK_MEM_QUANTUM to 4096")
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>