ShellPkg: Refine the code to avoid use a constant number as offset.

ShellPkg: Refine the code to avoid use a constant number as offset.

Refine the code to use 'sizeof(EFI_LOAD_OPTION)' as offset in buffer to get 'description' instead of using a constant number.
This change makes the code more readable.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19746 6f19259b-4bc3-4df7-8a09-765794883524