fix: Update usage to be argocd-vault-plugin

[werne2j]

Description

Update usage from argocd-vault to argocd-vault-plugin

Checklist

Please make sure that your PR fulfills the following requirements:

• Reviewed the guidelines for contributing to this repository
• The commit message follows the Conventional Commits Guidelines.
• Tests for the changes have been
• Docs have been added / updated

Type of Change

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

[codecov]

Codecov Report

Merging #56 (947e348) into main (5fe0e85) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main      #56   +/-   ##
=======================================
  Coverage   44.73%   44.73%           
=======================================
  Files          13       13           
  Lines         427      427           
=======================================
  Hits          191      191           
  Misses        213      213           
  Partials       23       23           

Impacted Files	Coverage Δ
cmd/root.go	0.00% <0.00%> (ø)

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[jkayani]

This is correct: the binary is published with the name argocd-vault-plugin

Nothing should break in terms of functionality, since this is just the usage message