ShellPkg/Ping6: Add a note in help for TargetIp

ShellPkg/Ping6: Add a note in help for TargetIp

'Ping6 TargetIp' is a valid command input but we bail out
when TargetIp is a link local address because we would want
the user to specify the NIC (via SourceIp) through which
he/she would want to ping to TargetIp. This patch updates
the help text to mention the same.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hegde Nagaraj P <nagaraj-p.hegde@hpe.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>