fixed "birthday" in from date to date-time

[hongwei1]

https://json-schema.org/understanding-json-schema/about.html#about

in the json body, it is
"birthday": "22-02-1732",
But in json schema, it is wrong. No time info in the birthday
"birthday": { "type": "string", "format": "date-time" },

So I changed it to :
"birthday": { "type": "string", "format": "date" },

[mdboom]

Thanks for the submission. date is actually not part of Draft 4, so it's actually correct for the current version of the document. I missed this in my updates for Draft 6, since I don't think it's in the release notes. I think what I'll do is merge this into that branch and update it to note that it's a new feature in Draft 6. (I'm trying to keep the document usable for those using Draft 4 and above).

[mdboom]

Oops. Wait a minute. I think this isn't even in Draft 6, only Draft 7 (which explains why it's not in the Draft 6 release notes). I'll address this (and merge this PR) as part of #79.