Updating the text on vocabulary hashes

[iherman]

This is to handle #1442. It makes it clearer that all references are on JSON-LD.

Two notes for the future:

• If this PR is accepted and merged, a similar change should be done for the DI spec's corresponding tables for consistency (including the typesetting style for the hash values).
• When going to Proposed REC, the URL for the JSON-LD versions must be changed in the text to use the final URL on W3C Date Space.

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