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