r.flowaccumulation: Rename flowaccumulation_diff to flow_accum_diff i…

…n the manual (#958)

src/raster/r.flowaccumulation/r.flowaccumulation.html

@@ -46,18 +46,17 @@ <h2>EXAMPLES</h2>
 r.colors map=flow_accum_new raster=flow_accum
 
 # check difference between flow_accum and flow_accum_new
-r.mapcalc expression="flowaccumulation_diff=if(flow_accum-flow_accum_new, flow_accum-flow_accum_new, null())"
+r.mapcalc expression="flow_accum_diff=if(flow_accum-flow_accum_new, flow_accum-flow_accum_new, null())"
 </pre></div>
 
 <img src="r_flowaccumulation_nc_example.png">
 
-<p>For some reason, there are slight differences between the two output maps.
-The yellow and purple cells show the difference raster map
-(<i>flowaccumulation_diff</i>). The red arrows and numbers represent drainage
-directions (<i>drain_directions</i>) and flow accumulation by
-<em>r.watershed</em> (<i>flow_accum</i>), respectively. Note that some cells
-close to headwater cells are assigned 1 even though they are located downstream
-of other cells.
+<p>There are slight differences between the two output maps. The yellow and
+purple cells show the difference raster map (<i>flow_accum_diff</i>). The red
+arrows and numbers represent drainage directions (<i>drain_directions</i>) and
+flow accumulation by <em>r.watershed</em> (<i>flow_accum</i>), respectively.
+Note that some cells close to headwater cells are assigned 1 even though they
+are located downstream of other cells.
 
 <p><img src="r_flowaccumulation_r_watershed_nc_example.png">