Fix incorrect bash_command example

[ralichkov]

Summary

One-word fix in one of the core doc pages. The echo command is missing unlike the other examples so I'm just adding it in.

Motivation

Copy-pasting the example blew up in my terminal.

[boring-cyborg]

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[ralichkov]

Forgot I had to manually the from airflow.sdk import DAG and the BashOperator so I might just double-check later and reopen.