fix integration test failure slack message

[karawoo]

The top-level text at line 73 is apparently only a fallback. I was expecting it to appear in the message but it doesn't; this PR adds the appropriate text in the body of the message.

Checklist

• Does this change update NEWS.md (referencing the connected issue if necessary)?
• Does this change need documentation? Have you run devtools::document()?
• Does this change resolve integration test failures for a preview build of Connect? If so (and if you are on the Connect team), please be sure to update the Connect changelog to reflect that users will need to upgrade their version of connectapi.

[jonkeane]

Thanks for this!