Fix documentation to reference MemoryManager

Per zendframework/zf2#6554, this document was improperly referring to each of
`Zend\Memory\Memory` and `Zend\Memory\Manager`; neither exist, but
`Zend\Memory\MemoryManager` _does_. Additionally, there is no `factory()`
method for creating a `MemoryManager()` instance; there is only a constructor.
That constructor also does not auto-create a cache backend for you; instead,
it expects a `Zend\Cache\Storage\StorageInterface` as an optional argument.

Documentation has been updated to properly detail the correct class, as well
as instantiation.