E070 should refer to "[extension, description]"

[srerickson]

In draft/spec/validation-codes.md, E070:

'If present, [the ocfl_layout.json document] MUST include the following two keys in the root JSON object: [key, description]'

Apologies if I'm misunderstanding the bracket syntax, but I believe this should read:

'If present, [the ocfl_layout.json document] MUST include the following two keys in the root JSON object: [extension, description]'