[freeciv-data] (PR#2406) In help, Rivers are indexed under mountains

I think this is the wrong approach. All three (rivers, terrain
alterations and pillaging) are special. And all three should be 
indented by another column. The attached patch fixes this.

The alternative would be to rename "Terrain" into "Terrain types" and 
add these three under a new item "Terrain".

    Raimar


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