enc.4: Remove a redundant word

Reviewed by: zlei
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D45706

(cherry picked from commit 4ca4a3b1c8e33cafcd9462e2d068246153846d57)
(cherry picked from commit d25d517db2e6b21753de39c68fd440c62dfe58d6)