docs: Update getting_started.md (cherry-pick #12718)

[gcp-cherry-pick-bot]

Cherry-picked docs: add namespace to initial-password command (#12718)

Added " -n argocd " to avoid unexpected error.

Signed-off-by: Devarsh devarshshah2608@gmail.com

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (1005183) 47.65% compared to head (1005183) 47.65%.

❗ Current head 1005183 differs from pull request most recent head d01684c. Consider uploading reports for the commit d01684c to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##           release-2.6   #12737   +/-   ##
============================================
  Coverage        47.65%   47.65%           
============================================
  Files              246      246           
  Lines            41699    41699           
============================================
  Hits             19873    19873           
  Misses           19839    19839           
  Partials          1987     1987           

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