Fix max health for entity

[ImmRanneft]

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Pull Request description

It's not allowed to have entities with health greater than 20, it could be found in Attribute class in Pocketmine-MP repo.
So this PR should fix that.

[kniffo80]

I would approve - but it's not possible 😄

[Sandertv]

+1

[jasonw4331]

+1