gshsec.8: Fix list alignment, tag spdx

MFC after:	3 days

lib/geom/shsec/gshsec.8

@@ -1,3 +1,6 @@
+.\"
+.\" SPDX-License-Identifier: BSD-2-Clause
+.\"
 .\" Copyright (c) 2005 Pawel Jakub Dawidek <pjd@FreeBSD.org>
 .\" All rights reserved.
 .\"
@@ -64,7 +67,7 @@ the rest of them.
 The first argument to
 .Nm
 indicates an action to be performed:
-.Bl -tag -width ".Cm destroy"
+.Bl -tag -width indent
 .It Cm label
 Set up a shared secret device from the given components with the specified
 .Ar name .
@@ -92,7 +95,7 @@ See
 .El
 .Pp
 Additional options:
-.Bl -tag -width ".Fl f"
+.Bl -tag -width indent
 .It Fl f
 Force the removal of the specified shared secret device.
 .It Fl h