Fix notifications action variable

[optik-aper]

trafficstars

Description

Corrected variable/input for mattermost action notification

Related Issues

#441

Checklist:

• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [ ] Have you linted your code locally prior to submission?
• [ ] Have you successfully ran tests with your changes locally?

[github-actions[bot]]

Unit Tests and Coverage

Show Output

?   	github.com/vultr/vultr-cli/v3	[no test files]

Pusher: @optik-aper, Action: pull_request_target

[optik-aper]

Superseded by #445