fix(ld): correct syntax for the get_numpy_matrix static method

[tskir]

✨ Context

Finemapping relies on the ld module to perform some operations. This module has an error in its static method, get_numpy_matrix.

🛠 What does this PR implement

This is a simple fix to ensure the static method works. Note that this is not the most elegant possible solution; it is possible that a static method could be replaced with a better approach, but this is outside the scope of this simple fix PR.

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