Shellpkg: Fix spelling errors in Shell.c/Shell.h comments
    
    This patch fixes spelling errors in comments. No functional change.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Joe Peterson <joe.peterson@intel.com>
    Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
    
    git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16816 6f19259b-4bc3-4df7-8a09-765794883524