Static virtual machine files should use YAML, not JSON

Static virtual machines files must be written in JSON format, thanks to https://github.com/GoogleCloudPlatform/PerfKitBenchmarker/blob/master/perfkitbenchmarker/static_virtual_machine.py#L187 . But the README documents YAML, not JSON, and given that other PKB configs also use YAML, it would make more sense for static VM files to be YAML too.