Fix example curl command for adding a user to a group/project
What does this MR do?
Fixes the example curl command for adding a user to a group/project - user_id should be part of POST data, not URL.
Notes:
- This also needs to occur in the CE documentation
- There is an inconsistency in how examples are formatted w.r.t. parameter specifications (e.g. :user_id) vs. values (e.g. 30 instead of :access_level). I didn't change this in my fix, but the examples should adopt a consistent usage pattern.