docs: `grouptag` not marked as optional although some senseboxes have no grouptags

[hf-kklein]

Expected Behavior

The API docs should describe the API correctly.
The grouptag should be marked as optional, because not all senseboxes have grouptags.
(Example: https://api.opensensemap.org/boxes/621f53cdb527de001b06ad5e)

Current Behavior

The grouptag is not marked as optional.

Possible Solution

Fix documentation.

Steps to Reproduce (for bugs)

Visit https://docs.opensensemap.org/#api-Boxes-getBox