Change subject of slack notifications

[iranzo]

Change subject of slack notifications

Signed-off-by: Pablo Iranzo Gómez Pablo.Iranzo@gmail.com

What this PR does / why we need it:

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[kubevirt-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[kubevirt-bot]

@iranzo: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
kubevirt-io-presubmit-link-checker	4e155ef	link	/test kubevirt-io-presubmit-link-checker

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