Documentation outdated for numpy example

[chhenning]

On the page I believe the example for initializing a Matrix with a numpy array is using the outdated "init" notation. Look for the "/* Bind MatrixXd (or some other Eigen type) to Python */" comment to find the code.

When I run the code I'm greeted with the following warning:

FutureWarning: pybind11-bound class 'chh.Matrix' is using an old-style placement-new '__init__' which has been deprecated. See the upgrade guide in pybind11's docs. This message is only visible when compiled in debug mode.

[YannickJadoul]

Thanks for reporting, @chhenning!