Fix supported dimension for VectorFEBoundaryTangentLFIntegrator

[mlstowell]

The documentation for VectorFEBoundaryTangentLFIntegrator in https://mfem.org/lininteg/ states that it supports 2D and 3D domains but this is not correct. The integrator itself contains an MFEM_VERIFY restricting its use to 2D elements on the boundary of a 3D domain.

Note: this was noticed as part of issue mfem/mfem#4114