Update earthworking tutorials

[baharmon]

Fixes issues in CLI commands raised by @micha-silver in baharmon/r.earthworks#25 for earthworking tutorials.

I came across two small problems with the tutorials:
In Gully Modeling, under the Fractal Terrain section, I believe that the accumulation=accumulation parameter should be removed. That (input) map does not yet exist, until after running r.watershed in the following code snippet.
In Modeling Coastal Infrastructure, I believe that --overwrite needs to be added in the Flood Simulation section and later in the final Flood Simulation section, since the lake raster exists in the original data.