DOC: Correct typos

Fix reported typos

closes #616

doc/source/iv/estimators.lyx

@@ -170,7 +170,7 @@ x_{2i} & =\underset{\textrm{exogenous}}{\underbrace{\gamma_{1}^{\prime}z_{1i}}}+
 The model can be expressed compactly 
 \begin_inset Formula 
 \begin{align*}
-Y & =X_{1}\beta_{1}+X_{2}\beta_{1}+\epsilon=X\beta+\epsilon\\
+Y & =X_{1}\beta_{1}+X_{2}\beta_{2}+\epsilon=X\beta+\epsilon\\
 X_{2} & =Z_{1}\gamma_{1}+Z_{2}\gamma_{2}+u=Z\gamma+u
 \end{align*}
 
@@ -218,7 +218,7 @@ The vector of instruments
 \begin_inset Formula $k$
 \end_inset
 
- matrix if regressors where row 
+ matrix of regressors where row
 \begin_inset Formula $i$
 \end_inset
 

doc/source/iv/estimators.txt

@@ -32,14 +32,14 @@ The model can be expressed compactly
 .. math::
 
    \begin{aligned}
-   Y & =X_{1}\beta_{1}+X_{2}\beta_{1}+\epsilon=X\beta+\epsilon\\
+   Y & =X_{1}\beta_{1}+X_{2}\beta_{2}+\epsilon=X\beta+\epsilon\\
    X_{2} & =Z_{1}\gamma_{1}+Z_{2}\gamma_{2}+u=Z\gamma+u\end{aligned}
 
 The vector of instruments :math:`z_{i}` is :math:`p` by 1. There are
 :math:`n` observations for all variables. :math:`k_{c}=1` if the model
 contains a constant (either explicit or implicit, i.e., including all
 categories of a dummy variable). The constant, if included, is in
-:math:`x_{1i}`. :math:`X` is the :math:`n` by :math:`k` matrix if
+:math:`x_{1i}`. :math:`X` is the :math:`n` by :math:`k` matrix of
 regressors where row :math:`i` of :math:`X` is :math:`x_{i}^{\prime}`.
 :math:`X` can be partitioned into :math:`\left[X_{1}\;X_{2}\right]`.
 :math:`Z` is the :math:`n` by :math:`p` matrix of instruments. The