[ci skip] Fix doc example in Mapper#each
[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.

cored/rom,jeremyf/rom,endash/rom,rom-rb/rom,vrish88/rom,denyago/rom,pdswan/rom,Snuff/rom,rom-rb/rom,kwando/rom,pvcarrera/rom,dekz/rom,pxlpnk/rom,dcarral/rom,rom-rb/rom