iw: event: add the missing time display format in the "iw event" help menu

The option used to print the events with timestamp in Human readable format
is not listed in the "$ iw event -h" output.

 $ ./iw event -h
 Usage:  ./iw [options] event [-t|-r] [-f] [-n]
 ...

So add "-T" option to the help menu.

 $ ./iw event -h
 Usage:  ./iw [options] event [-t|-T|-r] [-f] [-n]
 ...

Signed-off-by: Gokul Sivakumar <gokulkumar792@gmail.com>
Link: https://lore.kernel.org/r/20210910141618.1594617-3-gokulkumar792@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>