docs: correct integrate command in tutorial

[james-garner-canonical]

The juju integrate command in the tutorial will fail with an error because multiple interfaces are supported between the two charms.

Checklist

• Code style: imports ordered, good names, simple structure, etc
• Comments saying why design decisions were made
• Go unit tests, with comments saying what you're testing
• Integration tests, with comments saying what you're testing
• doc.go added or updated in changed packages

QA steps

Docs should build. Tutorial steps should work as documented.

ubuntu@my-juju-vm:~$ juju integrate self-signed-certificates postgresql-k8s
ERROR ambiguous relation: "self-signed-certificates postgresql-k8s" could refer
to "postgresql-k8s:certificates self-signed-certificates:certificates";
"postgresql-k8s:receive-ca-cert self-signed-certificates:send-ca-cert"
ubuntu@my-juju-vm:~$ juju integrate postgresql-k8s:certificates
self-signed-certificates:certificates

Documentation changes

This PR updates the tutorial to provide the unambiguous juju integrate command and explain why it's needed.

Links

Thanks to @marqode for reporting this issue and identifying the fix.

[james-garner-canonical]

Apologies if this PR is against the wrong branch -- I believe both the tutorial at /latest and at /3.6 need this fix.