Fix minor typos in BaseLib LinkedList comments

Signed-off-by: Samer El-Haj-Mahmoud elhaj@hp.com 
Reviewed-by: Liming Gao <liming.gao@intel.com>

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14204 6f19259b-4bc3-4df7-8a09-765794883524