Changed some headings.

docs/source/examples_simple_2D.rst

@@ -1,5 +1,5 @@
 ************************
-Introductory models - 2D
+Introductory (2D) models
 ************************
 
 This section provides some introductory example models in 2D that demonstrate basic features of gprMax. Each example comes with an input file which you can download and run.

docs/source/index.rst

@@ -59,6 +59,7 @@ gprMax User Guide
 
 .. toctree::
     :maxdepth: 2
+    :caption: Appendices
 
     references