fix: path error in docs for argo ui contributing

[reginapizza]

Small error in the path in this part of the docs I added, after going through this process myself I realized my error here. yarn add command should be run from the ui folder.

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).
• 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.
• Optional. My organization is added to USERS.md.
• 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).

[codecov]

Codecov Report

Merging #5898 (4d21b90) into master (6f5f87b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5898   +/-   ##
=======================================
  Coverage   40.89%   40.89%           
=======================================
  Files         147      147           
  Lines       19633    19633           
=======================================
  Hits         8028     8028           
  Misses      10500    10500           
  Partials     1105     1105           

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[jannfis]

LGTM, thank you!