target: fix syntax in help message

target: fix syntax in help message

The name of the command is already printed before the string
in ->usage, thus it appears twice.
E.g.
	> help examine
	  stm32f4x.cpu arp_examine arp_examine ['allow-defer']
	        used internally for reset processing
	  stm32f4x.cpu examine_deferred examine_deferred
	        used internally for reset processing
	  stm32f4x.cpu was_examined was_examined
	        used internally for reset processing

Remove the command name from the string in ->usage.

Change-Id: If3b1368ffff8a94eb629ae3679e2e5f2f11ae92e
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4536
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>