[AIRFLOW-6639] Remove duplicate Output format choices from CLI docs

[kaxil]

Valid values are: (fancy_grid|github|grid|html|jira|latex|latex_booktabs|latex_raw|mediawiki|moinmoin|orgtbl|pipe|plain|presto|psql|rst|simple|textile|tsv|youtrack)

The above is repeated twice. https://airflow.readthedocs.io/en/latest/cli-ref.html#tasks

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Issue link: AIRFLOW-6639

Make sure to mark the boxes below before creating PR: [x]

• Description above provides context of the change
• Commit message/PR title starts with [AIRFLOW-NNNN]. AIRFLOW-NNNN = JIRA ID^*
• Unit tests coverage for changes (not needed for documentation changes)
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
• I will engage committers as explained in Contribution Workflow Example.

^* For document-only changes commit message can start with [AIRFLOW-XXXX].

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In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.
Read the Pull Request Guidelines for more information.

[codecov-io]

Codecov Report

Merging #7259 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7259   +/-   ##
=======================================
  Coverage   85.33%   85.33%           
=======================================
  Files         791      791           
  Lines       40137    40137           
=======================================
  Hits        34252    34252           
  Misses       5885     5885

Impacted Files	Coverage Δ
airflow/bin/cli.py	94.73% <ø> (ø)	⬆️

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