tcp_ecn is an integer not a boolean

There was some confusion at LCA as to why the sysctl tcp_ecn took one
of three values when it was documented as a Boolean.  This patch fixes
the documentation.

Signed-off-by: Peter Chubb <peter.chubb@nicta.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>