dijkstra.py: Use r"strings" to fix two pylint warnings (#1052)

```
=============================== warnings summary ===============================
graphs/dijkstra.py:81
  /home/travis/build/TheAlgorithms/Python/graphs/dijkstra.py:81: DeprecationWarning: invalid escape sequence \ 
    """
graphs/dijkstra.py:97
  /home/travis/build/TheAlgorithms/Python/graphs/dijkstra.py:97: DeprecationWarning: invalid escape sequence \ 
    """
-- Docs: https://docs.pytest.org/en/latest/warnings.html
=================== 126 passed, 7 warnings in 19.35 seconds ====================
```