time.3: Update ERRORS section

time() is now implemented using clock_gettime(2) instead of
gettimeofday(2).

Reviewed by: debdrup
Fixes: 358ed16f7505 Use clock_gettime(CLOCK_SECOND)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34438

(cherry picked from commit 413045a52c5f14ac22516b4f622d0132dc34e882)