graph: update mermaid diagram

Author: Mizux

ortools/graph/docs/shortest_path_dijkstra.md

@@ -25,8 +25,8 @@ graph LR
     4((4))
     5((5))
     0 == "3" ==>1
-    0 -- "5" -->2
     1 -- "1" -->2
+    0 -- "5" -->2
     1 -- "4" -->3
     1 == "0" ==>4
     2 -- "2" -->4