Match the comments with the implementation for Baselib::GetFirstNode.

Signed-off-by: Shumin Qiu <shumin.qiu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdePkg@14532 6f19259b-4bc3-4df7-8a09-765794883524