Fix refget-openapi.yaml's Range header description

[jmarshall]

PR #331 noted that the text for Range in the YAML file was inconsistent with that in the specification.

While it was a Google Docs document, the specification was corrected to reflect that HTTP Range byte ranges are zero-based fully-inclusive. Update the OpenAPI description correspondingly.

Also fix typos.

[jmarshall]

Also: the YAML file mentions a /ping endpoint. This is not described in the spec document. Should it be added to the spec or removed from the YAML (perhaps because it has been superseded by /sequence/service-info)?

[andrewyatz]

Quite right about /ping. It's a custom endpoint I had on my implementation (the original YAML was based on my server version). It should be removed and I will get that going now