chore(ci): fix `deploy-docs` CI job

One step in the `deploy-docs` CI job contains a typo that causes it to
fail: `yarn -cwd ...` instead of `yarn --cwd ...`
This has been broken since a0488b30a7faa5c84998e4b276388dd839d1a761, but
has not been noticed because the job was not running. #17060 configured
the job to run as necessary, which brought up the error.

Example failure:
  - On v1.8.x:
    https://app.circleci.com/pipelines/github/angular/angular.js/
    153/workflows/6a9826ac-d191-4042-8c39-0c969c81e381/jobs/1606

This commit fixes the typo in the command.