r.grow.distance manual: set color table in example

After change to the `viridis` color table as the default, here the `rainbow` color table must be set explicitly in order to reproduce the example.

raster/r.grow.distance/r.grow.distance.html

@@ -72,6 +72,7 @@ North Carolina sample dataset:
 <div class="code"><pre>
 g.region raster=streams_derived -p
 r.grow.distance input=streams_derived distance=dist_from_streams
+r.colors map=dist_from_streams color=rainbow
 </pre></div>
 
 <div align="center" style="margin: 10px">
@@ -90,6 +91,7 @@ r.grow.distance input=streams_derived distance=dist_from_streams
 <div class="code"><pre>
 g.region raster=sea -p
 r.grow.distance -m input=sea distance=dist_from_sea_geodetic metric=geodesic
+r.colors map=dist_from_sea_geodetic color=rainbow
 </pre></div>
 
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