accounts spec uses dids with invalid syntax ('@' char)

[gobengo]

Context

• reviewing 'accounts' spec https://github.com/web3-storage/specs/blob/main/w3-account.md
• the 'w3-space' spec essentially depends on this too

Problem

• both the specs document using DIDs like did:mailto:alice@web.mail, and this string is not a valid did because it contains the '@' char in the method-specific-id section
  • see did-core #did-syntax
  • here is an issue where I confirmed with w3c ccg that unencoded @ is not allowed - w3c/did-core#829 (comment)

Possible Solutions:

• we acknowledge that @ is not allowed in DIDs, and that it must be percent-encoded (to %40) when in a did string
  • we update our specs to use this form of valid did string

[gobengo]

@Gozala wdyt. If you agree this is a concern, I can prepare a PR to specs to have examples only use DIDs with a percent-encoded @ char