repo.readonly_session has parameter snapshot, not snapshot_id

[DahnJ]

The docs imply that repo.readonly_session has a parameter snapshot_id, when in fact the parameter is named snapshot.

In Version Control:

session = repo.readonly_session(snapshot_id="BSHY7B1AGAPWQC14Q18G")

In Quickstart, there's a related syntax error:

earlier_session = repo.readonly_session(snapshot=snapshot_id=hist[1].id)

While this is a docs issue, I would also consider whether the parameter name should be unified in the API:

• snapshot is used for readonly_session, ancestry and async_ancestry methods
• snapshot_id is used elsewhere

If it's helpful, I am happy to make a PR with whichever change you think is best.