tcp.fastopen.client_enable: Fix documented default

The default value has been 1 since June 2018, but the docs were not
updated to reflect the change.

MFC after: 3 days
Reviewed by: ziaee
Fixes: af4da5865557 (Enable TCP_FASTOPEN by default)
Signed-off-by: Matteo Riondato <matteo@FreeBSD.org>
Closes: https://github.com/freebsd/freebsd-src/pull/2285

(cherry picked from commit 0bc852cc62afcd79fc776f784623c3f2b7691d27)