ShellPkg/TftpDynamicCommand: Clarify the retry count option in command.

ShellPkg/TftpDynamicCommand: Clarify the retry count option in command.

[-c <retry count>] is to define the number of times to transmit request
packets and wait for a response. The default value is 6. But it doesn't
specify the behavior of zero value. Here, The patch is to clear that:
Set to zero also means to use the default value.

Cc: Carsey Jaben <jaben.carsey@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>