seq.1: Reset option list alignment + tag spdx

This option list was tabbed over 19 characters. style.mdoc(5) asks
that lists are set to the longest item, or if that's too long then
to use indent. Resetting it to indent buys us two lines at MANWIDTH
80, and three at 59.

MFC after: 3 days
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D48350