Fix up require for graphlib in bench script

Author: Chris Pettitt

package-lock.json

@@ -42,4 +42,4 @@
     "url": "https://github.com/dagrejs/dagre.git"
   },
   "license": "MIT"
-}
+}

package.json

@@ -3,7 +3,7 @@
 var Benchmark = require("benchmark"),
     sprintf = require("sprintf").sprintf;
 
-var Graph = require("graphlib").Graph,
+var Graph = require("@dagrejs/graphlib").Graph,
     rank = require("../lib/rank"),
     layout = require("..").layout;