Review comments of Panos on `ssi-standards.md

• transmitting and storing verifiable digital credentials VCs => spell out VCs.

• DIF has recently begun work on defining the specs => the DIF has recently begun work on defining the specs

• General Comment: Apart from the SSI Standards text (https://gitlab.grnet.gr/essif-lab/framework/-/blob/master/docs/ssi-standards.md), the other parts do not use dashes correctly; they only use hyphens instead of em dashes.