mrsas: update man page

update mrsas(4) since big-endian is supported since
e34a057ca6ebdf8e30ec8b0dc21d18eb450bf36a

Approved by: re (gjb)
Reviewed by: bdragon, gbe
Sponsored by: Eldorado Research Institute (eldorado.org.br)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28475

(cherry picked from commit 20e1817f9a68abd9a232367e04d2afeaf4a3c2f8)
(cherry picked from commit 467f2b64dbc3da664bf1808fd6fc0f8a7f2c6842)