[DOCS] Fix typo in Task Management API docs

[nattachai305]

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[cla-checker-service]

💚 CLA has been signed

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[jrodewig]

Thanks for catching this @nattachai305.

Before we can merge your changes, you'll need to sign the CLA here:
https://www.elastic.co/contributor-agreement

[nattachai305]

@jrodewig I've signed the CLA associated with my account. How should I do further?

[jrodewig]

Thanks @nattachai305. I'll get this merged for you.

[jrodewig]

@elasticmachine test this please.

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Task Management)