[AIRFLOW-6488] Add missing relation to op4

[loicbenoit]

Context: https://airflow.apache.org/docs/stable/concepts.html#concepts-operators, near line 401
In last "and equivalent to", it seems op4 should still depend on both op2 and op3.

op1 >> op2 >> op4
op1 >> op3 >> op4

and equivalent to:

op1.set_downstream([op2, op3])
op4.set_upstream([op2, op3]) #Expected this, but it's missing.

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Issue link: AIRFLOW-6488

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[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request!