Update make.jl

Author: boriskaus

docs/make.jl

@@ -103,7 +103,7 @@ makedocs(;
             "18 - Alpine data integration" =>  "man/Tutorial_AlpineData.md",
             "19 - Jura tutorial" =>  "man/Tutorial_Jura.md",
             "20 - 2D model setups" => "man/Tutorial_NumericalModel_2D.md",
-            "21 - 3D model setups" => "man/Tutorial_NumericalModel_3D.md"
+            "21 - 3D model setups" => "man/Tutorial_NumericalModel_3D.md",
             "22 - Build geometry from polygons" =>  "man/tutorial_Polygon_structures.md"
         ],
         "User Guide" => Any[