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.
Merge branch 'hotfix/zf2-6554'

Close #1335
Fixes zendframework/zf2#6554