Fix typo in nginx.adoc

[Hans-4]

This PR fixes the typo OpemEMS UI in the nginx documentation page.

I also thought about if we could update this part to include the following command for users less familiar with terminal commands, but i wanted to hear your opinion about this first.

. Start nginx:
+
[source,bash]
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sudo systemctl start nginx
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+
See `man nginx` and the documentation on https://nginx.org for more information.

[sfeilmeier]

I also thought about if we could update this part to include the following command for users less familiar with terminal commands, but i wanted to hear your opinion about this first.

Yes, absolutely. But in that situation NGINX should be already started, as it starts automatically after install. Maybe systemctl restart nginx would be better - or nginx -s reload.