LaTeX not rendering in boundary_conditions.md

[jbisits]

The last equation in the open boundary conditions documentation does not display. I think this is due to wrapping begin{align} ... \end{align} in \begin{equation} ... \end{equation}

Oceananigans.jl/docs/src/numerical_implementation/boundary_conditions.md

Lines 213 to 221 in 6c40d7e

	```math
	\begin{equation}
	\begin{align}
	u^{n+1}_{1jk} &\approx u^{n+1}_{3jk} + (u^{n+1}_{2jk} - u^{n+1}_{jk4}) / 2 + \mathcal{O}(\Delta x^2),\\
	&= u^\star_{1jk} - \Delta t \left(\boldsymbol{\nabla}p^{n+1}_{3jk} + (\boldsymbol{\nabla}p^{n+1}_{2jk} - \boldsymbol{\nabla}p^{n+1}_{4jk}) / 2\right) + \mathcal{O}(\Delta x^2),\\
	&\approx u^\star_{1jk} + \mathcal{O}(\Delta x^2) + \mathcal{O}(\Delta t).
	\end{align}
	\end{equation}
	```

Can either be fixed by align -> aligned or removing \begin{equation} ... \end{equation} (both options correctly render the equation in the markdown preview on github). I have opened a PR to make this change.

[glwagner]

thank you!