du(1): document --si option

The --si option (human-readable output with SI units based on
powers of 1000) was implemented but missing from both the SYNOPSIS
and the options list.

PR: 265199
Signed-off-by: Kit Dallege <xaum.io@gmail.com>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/2104

usr.bin/du/du.1

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd April 24, 2026
+.Dd June 9, 2026
 .Dt DU 1
 .Os
 .Sh NAME
@@ -36,7 +36,7 @@
 .Op Fl -libxo
 .Op Fl Aclnx
 .Op Fl H | L | P
-.Op Fl g | h | k | m
+.Op Fl g | h | k | m | -si
 .Op Fl a | s | d Ar depth
 .Op Fl B Ar blocksize
 .Op Fl I Ar mask
@@ -116,6 +116,11 @@ Display block counts in 1073741824-byte (1 GiB) blocks.
 output.
 Use unit suffixes: Byte, Kilobyte, Megabyte,
 Gigabyte, Terabyte and Petabyte based on powers of 1024.
+.It Fl -si
+.Dq Human-readable
+output.
+Use unit suffixes: Byte, Kilobyte, Megabyte,
+Gigabyte, Terabyte and Petabyte based on powers of 1000.
 .It Fl k
 Display block counts in 1024-byte (1 kiB) blocks.
 .It Fl l