Remove duplicated reference to fix warning

doc/source/examples/dem/small-scale-rotating-drum-postprocessing/small-scale-rotating-drum-postprocessing.rst

@@ -12,7 +12,7 @@ This is an example of how to post-process results obtained in the `Small scale r
 
 .. warning::
 
-  Details about installing the module or using it without installing it are available on this `documentation <../../../tools/postprocessing/postprocessing.py>`_.
+  Details about installing the module or using it without installing it are available on `here <../../../tools/postprocessing/postprocessing.py>`_.
 
 
 ----------------------------------