doc(Combinatorics): spell cliquefree as clique-free

[harahu]

This change was initially suggested to me by Codex, as part of #30621, but it made sense to do this as a free-standing PR, since this change requires some justification, and this PR provides just that.

The fact that Codex is encouraging this change does not mean much on its own, so I did my best to check the literature. Searching on Google Scholar yields a mere four results for "cliquefree" and 423 results for "clique-free". Based on these results, I think it is safe to say that "cliquefree" is quite idiosyncratic, and should be avoided.

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[github-actions]

PR summary dde1cfffb6

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).