Update rollback syntax in Testing doc

[danielmbarlow]

When using the suggested code from the documentation:

# For the `neo4j` gem
config.around do |example|
  ActiveGraph::Base.transaction do |tx|
    example.run
    tx.failure
  end
end

We get this error:

NoMethodError:
       undefined method `failure' for an instance of Neo4j::Driver::Internal::InternalTransaction

The syntax is apparently now tx.rollback, however when the transaction has already been rolled back we get this error

     Neo4j::Driver::Exceptions::ClientException:
       Can't rollback, transaction has been rolled back

This pull request changes the documention to recommend:

• Usage of the new syntax for rollback
• A check to see if the transaction is open before rolling it back

[danielmbarlow]

Let me know if there's anything you would like me to change in this little PR.

[klobuczek]

Thanks @danielmbarlow. I just looked at your PR this week and was just surprised about the condition if tx.open?. Did you have to do it because of some error? In the context of activegraph the transaction should be always open if in scope.