Fix the zpool_destroy_004_pos.ksh test

tests/sys/cddl/zfs/tests/cli_root/zpool_destroy/zpool_destroy_004_pos.ksh

• Fix a typo that prevented "zfs send" from running.

	  This typo was present in the first revision of the file.

• Reduce by a factor of about 2 the amount of data used in this

	  test.  It was excessive and let to long runtimes.

Sponsored by: Spectra Logic Corp