r.sun.daily: use word aggregation instead of naming all methods

src/raster/r.sun.daily/r.sun.daily.html

@@ -8,7 +8,7 @@ <h3>Output parameters explanation</h3>
 There are two basic options:
 <ul>
 <li>output series of maps (one for each day): options containing basename in their name</li>
-<li>output one map which is a sum/avg of the intermediate maps</li>
+<li>output one map which is an aggregation of the intermediate maps</li>
 </ul>
 
 You can choose any combination of parameters: e.g. total map of diffuse radiance and