MFC r347963:
Change ed(4), ep(4), and fxp(4) examples to em(4).
ed(4) and ep(4) will be removed in FreeBSD 13. fxp(4) remains
popular in older systems, but isn't as future proof as em(4).
Reviewed by: bz, jhb
Differential Revision: https://reviews.freebsd.org/D20311