Fix a typo spotted by @lucjonker

fixes #53

topofeatures/topofeatures.tex

@@ -44,7 +44,7 @@ \section{Topographic properties}%
 \label{fig:filter}
 \end{marginfigure}%
 \index{kernel filter}
-A filter is conceptually the same as a focal operation in map algebra (in GIS), as as a convolutional filter in computer vision.
+A filter is conceptually the same as a focal operation in map algebra (in GIS), or as a convolutional filter in computer vision.
 Observe that in Figure~\ref{fig:filter} a 3$\times$3 window is shown, but a 5$\times$5%
 % \marginnote{5$\times$5 filters can be used}
 or larger window could also be used; the size to use depends on the scale at which one wants to extract a property.