docs(azure-iot-mqtt-base): fix MqttBase code example in devdoc

[Delapouite]

Checklist

• I have read the [contribution guidelines] (https://github.com/Azure/azure-iot-sdk-node/blob/master/.github/CONTRIBUTING.md).
• I added or modified the existing tests to cover the change (we do not allow our test coverage to go down).
• If this is a modification that impacts the behavior of a public API
  • I edited the corresponding document in the devdoc folder and added or modified requirements.

Reference/Link to the issue solved with this PR (if any)

Description of the problem

Parenthesis were not balanced correctly

Description of the solution

[YoDaMa]

lgtm

[YoDaMa]

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