Fix publish instructions

[louiszawadzki]

What does this PR do?

After the yarn v3 migration, the command for publishing has changed from yarn publish to yarn npm publish.

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests
• [ ] Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• [ ] Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)
• [ ] If this PR is auto-generated, please make sure also to manually update the code related to the change