r.walk: fix typo in manual

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@72664 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.walk/r.walk.html

@@ -17,7 +17,7 @@ represent friction costs.
 
 <p>
 This function is similar to <em><a href="r.cost.html">r.cost</a></em>,
-but in addiction to a friction map, it considers an anisotropic travel
+but in addition to a friction map, it considers an anisotropic travel
 time due to the different walking speed associated with downhill and
 uphill movements.