doc: memory: Fix SYS_MEM_BLOCKS_DEFINE_STATIC description

Fix SYS_MEM_BLOCKS_DEFINE_STATIC() description.
Use a "memory blocks allocator" instead of "slab",
which is most probably was copy-pasted from
the previous "slab" chapter by mistake.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>