Corrected typographical error where "on" was used rather than "one".

This is the naming convention established in the standardized "template" workflow.

The `repository_dispatch` event allows triggering workflows via the GitHub API. This might be useful for triggering an
immediate check in multiple relevant repositories after an external change, or some automated process. Although we don't
have any specific need for this event at the moment, the event has no impact on the workflow, so there is no reason
against having it. It is the sort of thing that can end up being useful if it is already in consistently in place, but
not worth setting up on demand, since the effort to set it up is greater than the effort to trigger all the workflows
manually.

This will facilitate testing and review of modifications to the workflow.

Because the workflow requires access to repository secrets, and so will fail whenever triggered by an event from a fork,
a conditional is added to make it only run when the modifications are made within the `arduino/arduino-ide`
repository.

No functional difference, and neither is necessarily superior, but this is how it is in the "template", and so it must be
here as well.

When no name is provided for a matrix job, the workflow job is named according to the contents of
`jobs[].<job_id>.strategy.matrix[]`. That can result in some fairly cryptic job names when the matrix contains a complex
data structure as is the case here. We already have a string to uniquely identify each certificate to humans, which is
exactly what the `jobs[].<job_id>.name` property does for jobs, so it will be an improvement to name the jobs according
to that identifier.

No functional change, and neither is necessarily superior, but this is the formatting style either defined in the
"template", or by the repository's Prettier formatting configuration preferences, so it must be brought into compliance
here as well.

The "Check Certificates" workflow is configured to send a notification via Slack if a problem is found with a certificate.
TThis is currently posted to the `team_tooling` channel, but that is not necessarily always going to be the case, and for
every deployment of the workflow. So a less specific secret name is more universally applicable to serve all applications
of this "template" workflow.

At the time the workflow was written the authors of the `rtCamp/action-slack-notify` GitHub Actions action did not
provide a major version ref. This meant that it was necessary to pin the action to a specific version.

Since then, a few new releases have been made, meaning an outdated version of the action was in use as a consequence of
the pinning.

The action now offers a `v2` major ref. Use of this ref will cause the workflow to benefit from ongoing development to
the action up until such time as a new major release is made, at which time we would need to evaluate whether any changes
to the workflow are required by the breaking change that triggered the major release before updating the major ref
(e.g., `uses: rtCamp/action-slack-notify@v3`).

This will make it easier for the maintainers to sync fixes and improvements in either direction between the upstream
"template" workflow and its installation in this repository.

Co-authored-by: Alberto Iannaccone <a.iannaccone@arduino.cc>