ShellPkg: Update the error prompt for ping6 command.

ShellPkg: Update the error prompt for ping6 command.

when we use the ping6 command without configuring the network
interface, it should give correct prompt to users.

Cc: Hegde Nagaraj P <nagaraj-p.hegde@hpe.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Reviewed-by: Sriram Subramanian <sriram-s@hpe.com>
Reviewed-by: Hegde, Nagaraj P <nagaraj-p.hegde@hpe.com>
Reviewed-By: Wu Jiaxin <jiaxin.wu@intel.com>