[ci skip] Fix doc example in Mapper#each

This shows that we need to inject the environment
into Mapper#initialize too, in order to be able to
provide access to non-relational mappers for users

Most likely Mapper and Relation::Mapper will work
with two different Environment subclasses. While
the relation mappers need to access Relation::Graph,
the collection mappers need to access some simple
CollectionRegistry object.
Change Mapper#collection visibility to protected