doc: Clarify behavior when sysctl tcp_ecn = 1

Recent commit (commit 7e3a2dc5 doc: make the description of how tcp_ecn
works more explicit and clear ) clarified the behavior of tcp_ecn sysctl
variable but description is inconsistent. When requested by incoming conections,
ECN is enabled with not just tcp_ecn = 2 but also with tcp_ecn = 1.

This patch makes it clear that with tcp_ecn = 1, ECN is enabled when requested
by incoming connections.

Also fix spelling of 'incoming'.

Signed-off-by: Vijay Subramanian <subramanian.vijay@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>