Corrected some doc building errors.

docs/source/features.rst

@@ -131,7 +131,7 @@ This latter option is often referred to as dielectric smoothing and has been sho
 Perfectly Matched Layer (PML) boundary conditions
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-With increased research into quantitative information from GPR, it has become necessary for models to be able to have more efficient and better-performing Perfectly Matched Layer (PML) absorbing boundary conditions. Since 2005 gprMax has featured PML absorbing boundary conditions based on the uniaxial PML (UPML) [GED1998]_ formulation. A PML based on a recursive integration approach to the complex frequency shifted (CFS) PML [GIA2012]_ has been adopted in the new version of gprMax. A general formulation of this RIPML, which can be used to develop any order of PML, has been used to implement first and second order CFS stretching functions. One of the attractions of the RIPML is that it is easily applied as a correction to the field quantities after the complete FDTD grid has been updated using the standard FDTD update equations. 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. Additionally, since the RIPML is media agnostic it can be used without change to problems involving dispersive and anisotropic materials. For further details see the :ref:`PML commands section <pml>`.
+With increased research into quantitative information from GPR, it has become necessary for models to be able to have more efficient and better-performing Perfectly Matched Layer (PML) absorbing boundary conditions. Since 2005 gprMax has featured PML absorbing boundary conditions based on the uniaxial PML (UPML) [GED1998]_ formulation. A PML based on a recursive integration approach to the complex frequency shifted (CFS) PML [GIA2012]_ has been adopted in the new version of gprMax. A general formulation of this RIPML, which can be used to develop any order of PML, has been used to implement first and second order CFS stretching functions. One of the attractions of the RIPML is that it is easily applied as a correction to the field quantities after the complete FDTD grid has been updated using the standard FDTD update equations. 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. Additionally, since the RIPML is media agnostic it can be used without change to problems involving dispersive and anisotropic materials. For further details see the :ref:`PML commands section <pml-commands>`.
 
 Open source, robust, file formats
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