block_device doc: Fix supported volume types

[runephilosof-karnovgroup]

The documentation lists the supported volume types with _, but it should be -.

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[lbajolet-hashicorp]

Hi @runephilosof-karnovgroup,

Thanks for this fix, this looks good to me; may I ask you to sign our CLAs before we merge this, as we won't be able to because of legal constraints otherwise.

Also since you changed the documentation for this attribute, could you regenerate the doc partials from it? Running make generate should be enough to have it all up-to-date.

Thanks!

[runephilosof-karnovgroup]

Sorry, I won't sign the CLA (too much effort required to get that approved, for this tiny contribution).
It was just easier to create a PR than describe it in an issue.

[runephilosof-karnovgroup]

Converted to an issue #219