fix: docs wrong name

[vinckr]

The link to identity-data-model was still named identity-user-model thus broken.
I didnt change the name of the document cause I thought it might break something, but would be good to change it as well.
Let me know if I should go ahead and change it.

I tested all links now locally and it worked fine!
(btw first time i figured out how to build kratos docs locally :D, will include a how-to in the contribute to docs soon!)

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