mrsas: update man page
update mrsas(4) since big-endian is supported since
e34a057ca6ebdf8e30ec8b0dc21d18eb450bf36a
Reviewed by: bdragon, gbe
Sponsored by: Eldorado Research Institute (eldorado.org.br)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28475