Typo in Dijkstra's priority queue comparison function breaks Dijkstra

[cxcorp]

When creating the min priority queue, the Dijkstra function passes in return cost1, cost2 as the function, causing it to always return cost2: https://github.com/chen0040/js-graph-algorithms/blob/01b523553f2169923185cd0c39a434c5792e9458/src/jsgraphs.js#L938-L940

It should probably be cost1 - cost2, like:

diff --git a/src/jsgraphs.js b/src/jsgraphs.js
index 5409240..1392673 100644
--- a/src/jsgraphs.js
+++ b/src/jsgraphs.js
@@ -936,7 +936,7 @@ var jsgraphs = jsgraphs || {};
         this.edgeTo = [];
         this.cost = [];
         this.pq = new jss.IndexMinPQ(V, function(cost1, cost2){
-            return cost1, cost2;
+            return cost1 - cost2;
         });
         
         for(var v =0; v < V; ++v){