docs: fix installation command

[simonseyock]

No description provided.

[simonseyock]

@weskamm please merge this if possible, thanks 🙏

[tostamm]

There's a small but important error inside the main-page / README.md

layers:
  - name: some-name
    wmts_rest_legend_url: '{base_url}/wmts/legend/{layer_name}.png'

As luckily described correctly in mapproxy.yaml wmts_rest_legend_url has to be wmts_rest_legendurl without '_' before url.
I think it would be good to correct this for future users. Thanks!