DOC: Fix raw LaTeX math syntax and grammar in GARCH11 docstring

[###] Issue with current documentation:

In pymc.distributions.timeseries.GARCH11 docstring (pymc/distributions/timeseries.py#L783-L793):

class GARCH11(Distribution):
    r"""
    GARCH(1,1) with Normal innovations. The model is specified by.

    .. math::
        y_t \sim N(0, \sigma_t^2)

    .. math::
        \sigma_t^2 = \omega + lpha_1 * y_{t-1}^2 + eta_1 * \sigma_{t-1}^2

    where \sigma_t^2 (the error variance) follows a ARMA(1, 1) model.

1. Raw LaTeX \sigma_t^2 in prose text is missing reST :math: directive, rendering literally as \sigma_t^2 on the Sphinx docs site instead of math $\sigma_t^2$.
2. Grammatical typo: follows a ARMA(1, 1) should be follows an ARMA(1, 1).
3. Punctuation: specified by. before math directive should be specified by:.

However sorry for that this is very simple ....

Idea or request for content:

Update GARCH11 docstring in pymc/distributions/timeseries.py to use :math:\ \sigma_t^2 `` and correct prose grammar.