fix bad link in index.html

[TallTed]

Tagging @jandrieu @philarcher to increase the odds of substantive review...

Given that we specify the branch (which seems not to be such a good idea, from today's perspective), we should fix it to target the correct branch which was changed out-of-band (was master, is now main).

In this PR, I change the target from master to main. Alternatively,

github: {"repoURL": "https://github.com/w3c/did-use-cases", "branch": "main"},

could become

github: {"repoURL": "https://github.com/w3c/did-use-cases"},

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[philarcher]

Thanks Ted.

This, and other issues, will likely be taken up by the new DID WG that's now getting under way.