manual: correct v.surf.rst example (#3743)

Update broken user example with more meaningful thresholds.

Co-authored-by: Anna Petrasova <kratochanna@gmail.com>

vector/v.surf.rst/v.surf.rst.html

@@ -318,7 +318,7 @@ <h3>Usage of the where parameter</h3>
 <div class="code"><pre>
 g.region raster=elevation -p
 # random elevation extraction of 500 samplings
-r.random elevation vector_output=elevrand n=500
+r.random -s elevation vector=elevrand n=500
 v.info -c elevrand
 v.db.select elevrand
 
@@ -329,11 +329,14 @@ <h3>Usage of the where parameter</h3>
 d.rast elev_full
 d.vect elevrand
 
-# interpolation based on subset of points (only those over 1300m/asl)
-v.surf.rst elevrand zcol=value elevation=elev_partial where="value > 1300"
-r.colors elev_partial raster=elevation
+# compute univariate statistics with 1st/3rd quartiles
+v.db.univar -e elevrand column=value
+
+# interpolation based on subset of points (only those over 1st quartile)
+v.surf.rst input=elevrand zcolumn=value elevation=elev_partial npmin=100 where="value > 94.9"
+r.colors map=elev_partial raster=elevation
 d.rast elev_partial
-d.vect elevrand where="value > 1300"
+d.vect elevrand where="value > 94.9"
 </pre></div>
 
 <h2>REFERENCES</h2>