Update README.md

Author: Jakub-Radzik

README.md

@@ -19,6 +19,6 @@
  10. Lab10: Binary Search Tree `13/20`
  11. Lab11: Red-Black Tree,  2-3-4 Tree `12/20`
  12. Lab12: Disjoint sets, comparison of trees `12/20`
- 13. Lab13: Graphs `xx/20`
- 14. Lab14: Algorithms: Prim, Kruskal, Dijkstra `xx/20`
- 15. Lab15: Huffman Coding `xx/20`
+ 13. Lab13: Graphs `12/20`
+ 14. Lab14: Algorithms: Prim, Kruskal, Dijkstra `12/20`
+ 15. Lab15: Huffman Coding `without mark`