Fix equations rendering in split_explicit_timesteppers.jl (#5285)

Author: navidcy

src/Models/HydrostaticFreeSurfaceModels/SplitExplicitFreeSurfaces/split_explicit_timesteppers.jl

@@ -11,8 +11,8 @@ The equations are evolved as follows:
 ```math
 \\begin{gather}
 U^{m+1} = U^m - Δτ (∂_x η^m - G^U), \\\\
-V^{m+1} = V^m - Δτ (∂_y η^m - G^V).
-η^{m+1} = η^m - Δτ (∂_x U^{m+1} + ∂_y V^{m+1}), \\\\
+V^{m+1} = V^m - Δτ (∂_y η^m - G^V), \\\\
+η^{m+1} = η^m - Δτ (∂_x U^{m+1} + ∂_y V^{m+1}).
 \\end{gather}
 ```
 """