fix wrong example (trac https://trac.osgeo.org/grass/ticket/896)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@40755 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/simwe/r.sim.water/r.sim.water.html

@@ -128,7 +128,7 @@ Spearfish region:
 
 <div class="code"><pre>
 g.region rast=elevation.10m -p
-r.slope.aspect elevation=elevation.10m elev_dx=elev_dx elev_dy=elev_dy
+r.slope.aspect elevation=elevation.10m dx=elev_dx dy=elev_dy
 
 # synthetic maps
 r.mapcalc "rain    = if(elevation.10m, 5.0, null())"