Fix the configuration index name

[weierophinney]

Looks like the example code in bootstrapSession() expects to read a key sessionand not session_manager from the configuration.

Provide a narrative description of what you are trying to accomplish:

• [x] Documentation enhacement

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Originally posted by @ekarakashi at https://github.com/zendframework/zend-session/pull/124

[weierophinney]

We have ticket #71 to fix the whole documentation for Session Manager (I think we should at least mention default factory we have in the package).

Also I'd like to point https://github.com/zendframework/zend-session/pull/61 where we made a change.

/cc @froschdesign

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Originally posted by @michalbundyra at https://github.com/zendframework/zend-session/pull/124#issuecomment-544695940

[weierophinney]

@webimpress

I think we should at least mention default factory we have in the package

We must document the factory because this is the way the user should go. These manual configuration must be removed from all packages.

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Originally posted by @froschdesign at https://github.com/zendframework/zend-session/pull/124#issuecomment-544699438

[genby8]

'service_manager' => [
        'factories' => [
            SaveHandlerInterface::class => SaveHandlerDbFactory::class,
            SaveHandlerDbFactory::class => InvokableFactory::class,
        ]
]

namespace App\Session\SaveHandler;

use Interop\Container\ContainerInterface;
use Laminas\Db\Adapter\Adapter;
use Laminas\Db\TableGateway\TableGateway;
use Laminas\ServiceManager\Factory\FactoryInterface;
use Laminas\Session\SaveHandler\DbTableGateway;
use Laminas\Session\SaveHandler\DbTableGatewayOptions;

class SaveHandlerDbFactory implements FactoryInterface
{
    public function __invoke(ContainerInterface $container, $requestedName, array $options = null)
    {
        return new DbTableGateway(
            new TableGateway( 'session', $container->get(Adapter::class)),
            new DbTableGatewayOptions()
        );
    }
}

This best way inject?