i.vi: update manual after PVI changes (#2903)

imagery/i.vi/i.vi.html

@@ -262,14 +262,11 @@ <h3>Vegetation Indices</h3>
 <b>PVI: Perpendicular Vegetation Index</b>
 
 <div class="code"><pre>
-pvi( redchan, nirchan )
+pvi( redchan, nirchan, soil_line_slope )
 
 PVI = sin(a)NIR-cos(a)red
 </pre></div>
 
-for a isovegetation lines (lines of equal vegetation)
-would all be parallel to the soil line therefore <tt>a=1</tt>.
-
 <p>
 <b>SAVI: Soil Adjusted Vegetation Index</b>
 
@@ -437,7 +434,7 @@ <h3>Calculation of PVI</h3>
 
 <div class="code"><pre>
 g.region raster=band.3 -p
-i.vi red=band.3 nir=band.4 viname=pvi output=pvi
+i.vi red=band.3 nir=band.4 soil_line_slope=0.45 viname=pvi output=pvi
 r.univar -e pvi
 </pre></div>