Remove or edit redundant MUST statement

[julianmorley]

https://ocfl.io/draft/spec/#E065 is currently:

Every inventory file must have a corresponding Inventory Digest.

This seems to be covered by E058:

Every occurrence of an inventory file must have an accompanying sidecar file stating its digest.

As it stands a validation for inventory sidecars would raise both E058 and E065 if the sidecar file was not found. I think the offending E065 statement could just be removed entirely, or perhaps re-worded to refer back to the earlier statements.

[zimeon]

While I think we could just delete the sentence with E065, technically it is redundant, I think we miss an opportunity to remind readers about the digest. How about changing to a reference only:

See also requirements for the corresponding Inventory Digest.

[zimeon]

Agreement in editors' call to make change as above