fix Cat API example

[jeinwag]

The Cat API example in the overview of the docs does not work, since the Cat API returns plain text by default, so the deserialization into a JSON value will fail.

Explicitly telling the API to return JSON fixes this.

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[cla-checker-service[bot]]

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