man: Update the -# (--number) option entry

Remove "optional". If we choose to print a packet number, it is not
optional.

(cherry picked from commit 9d2e0f1)

[skip ci]

Author: fxlb

tcpdump.1.in

@@ -624,7 +624,7 @@ instead of ``nic.ddn.mil''.
 .TP
 .B \-\-number
 .PD
-Print an optional packet number at the beginning of the line.
+Print a packet number at the beginning of the line.
 .TP
 .B \-O
 .PD 0