r.sun: fix documentation of shadowed areas

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@71800 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.sun/r.sun.html

@@ -224,7 +224,7 @@ END OF WE DON'T KNOW
 
 <h3>Extraction of shadow maps</h3>
 A map of shadows can be extracted from the solar incidence angle map
-(incidout). Areas with zero values are shadowed. This will not work
+(incidout). Areas with NULL values are shadowed. This will not work
 if the <em>-p</em> flag has been used.
 
 <h3>Large maps and out of memory problems</h3>