docs: fix typo in unit test npm command

Description

Tiny change to fix a typo in a heading about unit tests.

The proper command is npm run test:unit, the readme says npm run test-unit.

Related Issues

Related to https://gitlab.com/gitlab-org/editor-extensions/meta/-/issues/229+

How has this been tested?

N/A

Screenshots (if appropriate)

No visual change

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes