docs/vm: numa_memory_policy: s/Linux memory policy/NUMA memory policy/

The document describes NUMA memory policy and as it is a part of the Linux
documentation it's obvious that this is Linux memory policy. Besides,
"Linux memory policy" may refer to other policies, e.g. memory hotplug
policy, and using term NUMA makes the documentation less ambiguous.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>