docs: Update argocd-cm.yaml bannerposition description (#16961)

[keithchong]

Fixes #16961

The cool feature #14623 was added. We need to update the sample argocd-cm.yaml file in the documentation.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (97727cb) 49.21% compared to head (898d907) 49.21%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #16962   +/-   ##
=======================================
  Coverage   49.21%   49.21%           
=======================================
  Files         274      274           
  Lines       48111    48111           
=======================================
  Hits        23677    23677           
  Misses      22090    22090           
  Partials     2344     2344           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ishitasequeira]

LGTM!!