Fix function header comments for DiskIoDestroySubtask.

Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdeModulePkg@14577 6f19259b-4bc3-4df7-8a09-765794883524