r.watershed manual: wording cosmetics

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@60854 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.watershed/front/r.watershed.html

@@ -56,7 +56,7 @@ values are valid elevation data.  Gaps in the elevation map that are
 located within the area of interest must be filled beforehand,
 e.g. with <em><a href="r.fillnulls.html">r.fillnulls</a></em>, to
 avoid distortions.  The elevation map need not be sink-filled because
-the module uses the least-cost algorithm.
+the module uses a least-cost algorithm.
 
 <p>
 Map of actual depressions or sinkholes in the landscape that are large