Update documentation to make the [world] argument more clear

[Ferrothrax]

The argument that is required for squaremap commands to define the world to operate on is not documented.
At first I tried to use the levelname from the server.properties file ("world" in my case) and received the [Server thread/INFO]: [squaremap] Invalid command argument: No such world 'world' result. Various other attempts likewise failed.
I found vague references to the need to use a "worldkey" in previous issues, but couldn't find any reference to what that actually was with any amount of Googling.
Only after digging down several rabbit holes did I find a screenshot on the Discord showing the map with "minecraft:overworld", "minecraft:the_nether", and "minecraft:the_end". Trying this finally produced the result I was looking for.

I think it would greatly help new users to document these on the project's wiki under the "Commands and Permissions" page.

[GuilhermeFaga]

Thanks for this issue. I too found the argument of this command very misleading.