Rewrite description of sidecar file to remove uncheckable MUST

[zimeon]

Per #444 we end up with 3 RFC2119 MUST for describing inventory sidecar files. I think we need only 2 MUST and propose changing the text to something like:

Every occurrence of an inventory file [E058]MUST have an accompanying sidecar file named inventory.json.ALGORITHM stating its digest, where ALGORITHM is the chosen digest algorithm for the object. The ALGORITHM [E059]MUST match the value given for the digestAlgorithm key in the inventory. An example might be inventory.json.sha512.

which has the effect of removing E092

[julianmorley]

There's also https://ocfl.io/draft/spec/#E065 a bit further down;

Every inventory file must have a corresponding Inventory Digest.

Which is also redundant.

[zimeon]

New issue for E065?

[julianmorley]

Done!
#449

[zimeon]

Agreement in editors' call to make change as above #448 (comment)