docs: Fix typo in Documentation/memory-barriers.txt

 (2) to provide a guide as to how to use the barriers that are available.

Note that an architecture can provide more than the minimum requirement

for any particular barrier, but if the architecure provides less than

for any particular barrier, but if the architecture provides less than

that, that architecture is incorrect.

Note also that it is possible that a barrier may be a no-op for an