Hessian Matrix formula is wrong

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From the manual:

The Hessian is the matrix of second order partial derivatives, so for 3x3 matrices and higher the matrix should still consist of $\dfrac{\partial^2}{\partial x_i \partial x_j}$. Currently for 3x3 hmat generates a matrix with elements such as $\dfrac{\partial^3}{\partial x \partial z}$, which is incorrect. The superscript in the numerator should always be 2 no matter the dimension.

From the wikipedia page:

Thanks for all your hard work on this package! It's now a must have for me when using Typst.

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#13

[Leedehai]

Fixed by that PR #13. Thanks!