documentation: Fix smp typo in memory-barriers.txt

      However, they do -not- guarantee any other sort of ordering:

      Not prior loads against later loads, nor prior stores against

      later anything.  If you need these other forms of ordering,

      use smb_rmb(), smp_wmb(), or, in the case of prior stores and

      use smp_rmb(), smp_wmb(), or, in the case of prior stores and

      later loads, smp_mb().

  (*) If both legs of the "if" statement begin with identical stores