nvmecontrol.8: Fix the SYNOPSIS section

The manpage of nvmecontrol(8) has the following SYNOPSIS:

nvmecontrol format [-f fmt] [-m mset] [-o pi] [-l pil] [-E]
[-C] <device-id | namespace-id>

The correct switch for the pi option is -p according
to sbin/nvmecontrol/format.c:

OPT("pi", 'p', arg_uint32, opt, pi, "Protective information")

So correct the SYNOPSIS section accordingly.

PR: 276554
Reviewed by: imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D44099