Don't forget self in table_formatter.py

Don't forget self in table_formatter.py

__ridx__ is an _undefined name_ in this context but __self.ridx__ is used three other times in this method and nine times in this class.  Undefined names may raise [NameError](https://docs.python.org/3/library/exceptions.html#NameError) at runtime.

flake8 testing of https://github.com/opencv/opencv

$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./modules/ts/misc/table_formatter.py:50:23: F821 undefined name 'ridx'
            self.rows[ridx + 1].props = properties
                      ^
```