Fix docs (#2460)

Author: huiyuxie

src/solvers/dgsem/dgsem.jl

@@ -14,8 +14,7 @@ include("basis_lobatto_legendre.jl")
     DGSEM(; RealT=Float64, polydeg::Integer,
             surface_flux=flux_central,
             surface_integral=SurfaceIntegralWeakForm(surface_flux),
-            volume_integral=VolumeIntegralWeakForm(),
-            mortar=MortarL2(basis))
+            volume_integral=VolumeIntegralWeakForm())
 
 Create a discontinuous Galerkin spectral element method (DGSEM) using a
 [`LobattoLegendreBasis`](@ref) with polynomials of degree `polydeg`.