Close XML element in `World.cs`

[limebell]

Following XML elements should be closed:

• https://github.com/planetarium/libplanet/blob/main/src/Libplanet.Action/State/World.cs#L34
• https://github.com/planetarium/libplanet/blob/main/src/Libplanet.Action/State/World.cs#L38
• https://github.com/planetarium/libplanet/blob/main/src/Libplanet.Action/State/World.cs#L55