fix: put back the test/skeleton/ring.dot file (break the tuto)

test/skeleton/ring.dot

+graph ring {
+ p1 [algo="some_algo.ml"]
+ p2 [algo="some_algo.ml"]
+ p3 [algo="some_algo.ml"]
+ p4 [algo="some_algo.ml"]
+ p5 [algo="some_algo.ml"]
+ p6 [algo="some_algo.ml"]
+ p7 [algo="some_algo.ml"]
+
+ p1 -- p2 -- p3 -- p4 -- p5 -- p6 -- p7 -- p1 
+}