Updating dead links in the contributing.md file.

INT-3443: Fix `CONTRIBUTING.md`

JIRA: https://jira.spring.io/browse/INT-3443

Removing the hashtag link from the URL as it wasn't being recognized by the browser.
INT-3429 AMQP Publisher Confirm Config Improvement

JIRA: https://jira.spring.io/browse/INT-3429

Require a `confirm-correlation-expression` when a
`confirm-ack-channel` or `confirm-nack-channel` is
specified.
INT-3430 AMQP Outbound: Add Eager Connect

JIRA: https://jira.spring.io/browse/INT-3430

Add an option to eagerly connect to rabbit when
only using outbound endpoints. Log an ERROR
if the connection can not be established during
context initialization.

Polishing