[12.x] Fix incorrect default retry info in Horizon docs

[OussamaMater]

This PR fixes a small misleading detail about how Horizon handles retries when the $tries option is omitted. In such cases, Horizon retries indefinitely, as opposed to the current docs which state it defaults to 1.

You can test this easily by:

• Creating a fresh Laravel app
• Installing Horizon
• Omitting the $tries option
• Dispatching any job that throws an exception in its handle() method

You will see that the job is retried indefinitely instead of being marked as failed after the first attempt.

For reference, here is where the default value is defined if no tries are set:

• https://github.com/laravel/horizon/blob/5.x/src/Console/SupervisorCommand.php#L36
• https://github.com/laravel/horizon/blob/5.x/src/SupervisorOptions.php#L213