make the example cube grid puzzle a possible one

[gordonwoodhull]

I got the example page working before I enabled the hexagonal playing field, so it currently shows an invalid, impossible cube grid puzzle.

Here is a selected random puzzle which has a solution.

[vercel[bot]]

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Name	Status	Preview	Comments	Updated (UTC)
hexapipes	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jun 11, 2023 3:03am

[gereleth]

Thanks, I never noticed!