Fix assertion message

[YichiZhang0613]

• I agree to follow the project's code of conduct.
• I added an entry to rstar/CHANGELOG.md if knowledge of this change could be valuable to users.

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The assertion requires MAX_SIZE >= 4 while the message requires MAX_SIZE > 4, so the message should be changed into "MAX_SIZE too small. Must be at least 4.".

[adamreichold]

Thanks! Could you run cargo fmt to update the formatting so the CI will pass?

[YichiZhang0613]

Thanks! Could you run cargo fmt to update the formatting so the CI will pass?

OK!