Fix 'healt' typo

[mathiasaerts]

Noticed this typo in the documentation so decided to create a pull request.

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[mathiasaerts]

Thank you for your pull request. Before we can look at it, you'll need to sign a Contributor License Agreement (CLA).

Please follow instructions at https://icinga.com/company/contributor-agreement to sign the CLA.

After that, please reply here with a comment and we'll verify.

Contributors that have not signed yet: @mathiasaerts

Seems a bit much for such a small change, but I just signed the CLA.

[bobapple]

@cla-bot check