zed: prettify slack notification message

[PandaScience]

Motivation and Context

Currently, when sending ZED notifications to slack, the message is a bit hard to read because it is formatted as plain text:

With this change, the body of the message is put in a monospaced code block which is far more convenient to read already because columns are properly aligned now:

How Has This Been Tested?

Well, there is not that much to test. I've edited the respective file on my local zfs installation and been using it with this config for some time now. The slack api is well documented and code block formatting is a standard thing in webhook payloads. You can verify the result in the screenshots above.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.