[Docs Bug 🐞 report]: Fix typos in add-server.md

[mscriven]

Describe the bug you found in AsyncAPI Docs.

There's a couple of typos in the Server reusability page.

The code reads:

servers:
  kafka-test:
    $ref: '#/components/servers/kafka-test
  mqtt-test:
    $ref: '#/components/servers/mqtt-test
components
  servers:
    kafka-test:
      host: my.kafka.pl
      protocol: kafka-secure
      description: Test Kafka server
    mqtt-test:
      host: test.mosquitto.org
      protocol: mqtt
      description: Test MQTT server

but it should be:

servers:
  kafka-test:
    $ref: '#/components/servers/kafka-test'
  mqtt-test:
    $ref: '#/components/servers/mqtt-test'
components:
  servers:
    kafka-test:
      host: my.kafka.pl
      protocol: kafka-secure
      description: Test Kafka server
    mqtt-test:
      host: test.mosquitto.org
      protocol: mqtt
      description: Test MQTT server

Attach any resources that can help us understand the issue.

Code is at URL: add-server#server-reusability

Image of the rendered typos:

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[mscriven]

Spotted while working locally, fixed and submitted a PR: #3171