cal: add description about option -V to manpage

cal support option -V to display version information, but the manpage doesn't
mention it.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>

misc-utils/cal.1

@@ -65,6 +65,8 @@ Display Monday as the first day of the week.
 Display Julian dates (days one-based, numbered from January 1).
 .It Fl y
 Display a calendar for the current year.
+.It Fl V
+Display version information and exit.
 .El
 .Pp
 A single parameter specifies the year (1 - 9999) to be displayed;