fixing additional (None) in finite_markov

[fkazemian]

(Issue #27) in the text of finite_markov there is an additional (None) which is not related to text.

This is not because of
If we think of :math:\psi_{t+1} and :math:\psi_t as *row vectors* (as is traditional in this literature), these :math:nequations are summarized by the matrix expression

this is because of the formula before that. It was something like

.. _mc_fdd:

.. math::

\psi_{t+1}(y) = \sum_{x \in S} P(x,y) \psi_t(x)

I add some space before .. math:: like

.. _mc_fdd:

    .. math::

    \psi_{t+1}(y) = \sum_{x \in S} P(x,y) \psi_t(x)

and that (None) disappeared.

[mmcky]

@fkazemian please add a newline between math directive and math

[fkazemian]

Hi @mmcky , I just updated this PR, would you please merge it? Thanks