Fix queue group link in Concurrent Message Processing example

[joeriddles]

Fix link for queue group in Concurrent Message Processing

Before:

By default, when a subscription is created, each message that is received it process sequentially. There can be multiple subscriptions setup in a [queue group][queue] in which case the NATS server will distribute messages to each member of the group.

After:

By default, when a subscription is created, each message that is received it process sequentially. There can be multiple subscriptions setup in a queue group in which case the NATS server will distribute messages to each member of the group.

[github-actions]

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[bruth]

Thanks!