Fix docstrings for the ShallowWaterEquations3D (#90)

* fix docstrings

* Apply suggestions from code review

Co-authored-by: Hendrik Ranocha <ranocha@users.noreply.github.com>

---------

Co-authored-by: Hendrik Ranocha <ranocha@users.noreply.github.com>

Author: patrickersing

src/equations/shallow_water_3d.jl

@@ -115,8 +115,9 @@ end
 end
 
 """
-    flux_wintermeyer_etal(u_ll, u_rr, orientation_or_normal_direction,
-                          equations::ShallowWaterEquations2D)
+    flux_wintermeyer_etal(u_ll, u_rr,
+                          normal_direction::AbstractVector,
+                          equations::ShallowWaterEquations3D)
 
 Total energy conservative (mathematical entropy for shallow water equations) split form.
 When the bottom topography is nonzero this scheme will be well-balanced when used as a `volume_flux`.
@@ -160,8 +161,9 @@ Further details are available in Theorem 1 of the paper:
 end
 
 """
-    flux_fjordholm_etal(u_ll, u_rr, orientation,
-                        equations::ShallowWaterEquations1D)
+    flux_fjordholm_etal(u_ll, u_rr,
+                        normal_direction::AbstractVector,
+                        equations::ShallowWaterEquations3D)
 
 Total energy conservative (mathematical entropy for shallow water equations). When the bottom topography
 is nonzero this should only be used as a surface flux otherwise the scheme will not be well-balanced.