block_device doc: Fix supported volume types

[runephilosof-karnovgroup]

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

packer-plugin-googlecompute/lib/common/block_device.go

Lines 95 to 98 in a85c050

	// * pd_standard: persistent, HDD-backed disk
	// * pd_balanced: persistent, SSD-backed disk
	// * pd_ssd: persistent, SSD-backed disk, with extra performance guarantees
	// * pd_extreme: persistent, fastest SSD-backed disk, with custom IOPS

But it should be

	// * pd-standard: persistent, HDD-backed disk
	// * pd-balanced: persistent, SSD-backed disk
	// * pd-ssd: persistent, SSD-backed disk, with extra performance guarantees
	// * pd-extreme: persistent, fastest SSD-backed disk, with custom IOPS