Regex error message in validate_query doesn't include periods

**Describe the bug**
The `validate_query` function in `backend/apps/core/validators.py` line uses the regex `r"^[\w .-]*$"` which allows periods ('.'). At first I thought this shouldn't even be allowed. But i realise it maybe the desired behavior. In that case, we should update the message to explicitly mention that periods are allowed.

**Are you going to work on fixing this?**

- [x] Yes
- [] No