Corrected some doc building errors.

docs/source/gprmodelling.rst

@@ -107,6 +107,6 @@ The absorbing boundary conditions (ABCs) employed in gprMax will, in general, pe
 
 The cells of the RIPML, which have a user adjustable thickness, very efficiently absorb most waves that propagate in them. Although, source and output points can be specified inside these cells **it is wrong to do so** from the point of view of correct modelling. The fields inside these cells are not of interest to GPR modelling. Placing sources inside these cells could have effects that have not been studied and will certainly provide erroneous results from a GPR modeller's point of view. The requirement to keep sources and targets at least 15 cells away for the PML has to be taken into account when deciding the size of the model domain. Additionally, free space (i.e. air) should be always included above a source for at least 15-20 cells in GPR models. Obviously, the more cells there are between observation points, sources, targets and the absorbing boundaries, the better the results will be.
 
-gprMax now offers the ability (for advanced users) to customise the parameters of the PML which allows its performance to be better optimised for specific applications. For further details see the :ref:`PML commands section <pml>`.
+gprMax now offers the ability (for advanced users) to customise the parameters of the PML which allows its performance to be better optimised for specific applications. For further details see the :ref:`PML commands section <pml-commands>`.
 
 This user guide, cannot serve as an in-depth tutorial and review of the FDTD method. However some useful hints and tips are given in order to cover the most fundamental aspects of using a FDTD based solver and to avoid the most common errors.