fix return type

Author: s-m-e

src/bewegung/linalg/_matrixarray.py

@@ -173,7 +173,7 @@ def __getitem__(self, idx: Union[int, slice]) -> Union[Matrix, MatrixArrayABC]:
 
         dtype = dtype_np2py(self.dtype)
 
-        return MatrixArray(
+        return Matrix(
             matrix = [
                 [dtype(col[idx]) for col in row]
                 for row in self._matrix