In v2.MetadataTest, `envVar` property is actually `env`

According to the patterns for converting the listed properties into YAML properties from the README.md, it seems like the `EnvVars` should correspond to a property named `envVars` but it is actually `env` (checking from the code at https://github.com/GoogleContainerTools/container-structure-test/blob/master/pkg/types/v2/metadata.go#L26). Should it be renamed to `envVars` or should the docs be changed to `env`?