Fix places_left CourseInvitation property

[Dmi4er4]

https://st.yandex-team.ru/DEVSHAD-174

Checklist:

• I have performed a self-review of my own code:
  • Good naming: make sure you would understand your code if you read it a few months from now.
  • No common performance issues (e.g. N + 1 problem)
  • Unrelated code has been removed (unused imports, print statements, unrelated template context variables, commented code, etc)
  • I have commented my code, particularly in hard-to-understand areas
• I understand that short PRs lead to higher-quality code and faster delivery of features.
• Linear history: my code has been rebased on the most recent commit in the master branch before opening PR
• I have followed conventional commits specification for PR title
• I have referenced YouTrack issue in the PR description for easy access
• I have added tests that prove my code works