Reference not rendering in Elliptic solvers section in the docs

[tomchor]

Pretty minor issue but \ref{sec:fractional-step} isn't rendering in the docs:

https://github.com/CliMA/Oceananigans.jl/blob/2edd2b1fa321725ad4e7891d3442ca9dd0657d98/docs/src/numerical_implementation/elliptic_solvers.md?plain=1#L46

I couldn't figure out what that's supposed to link to, so I couldn't fix it myself.