fix error in numpy_ml.linear_models.rst

[WrRan]

just fix error in numpy_ml.linear_models.rst

from

where (\mathbf{X}^\top \mathbf{X} + \alpha \mathbf{I})^{-1}
\mathbf{X}^\top is the pseudoinverse / Moore-Penrose inverse adjusted for the L2 penalty on the model coefficients.

to

where (\mathbf{X}^\top \mathbf{X} + \alpha \mathbf{I})^{-1}
is the pseudoinverse / Moore-Penrose inverse adjusted for the L2 penalty on the model coefficients.