libsysdecode: Fix shutdownhow table

MFC after: 1 week
Fixes: c3276e02beab ("sockets: make shutdown(2) how argument a enum")
Reviewed by: glebius
Differential Revision: https://reviews.freebsd.org/D57915

(cherry picked from commit 402a6574886a90b8025777b3b2c960c88677462d)