[AIRFLOW-XXXX] Update temp link to a fixed link

[kaxil]

http://airflow.apache.org/docs/stable/dag-serialization.html#limitations will be replaced when we have a new version.

So https://airflow.apache.org/docs/1.10.9/dag-serialization.html#limitations would be more appropriate

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Issue link: Document only change, no JIRA issue

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