docs(README): fix "Running Tests" instructions

[wesleyboar]

Description

Fix README.md "Running Tests" instructions.

Related resources

Problem found when trying to run test cases for https://github.com/django-cms/djangocms-bootstrap4/pull/145#issuecomment-1146662847.

Checklist

• [x] I have opened this pull request against master
• [ ] I have added or modified the tests when changing logic → N/A
• [x] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [x] I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request. → request