update iframe src

Author: fchasen

README.md

@@ -245,8 +245,8 @@ The final mapping is obtained when the equilibrium is reached.
 
 Here is an illustration of a dynamic graph layout based on a similar idea. Nodes are connected via springs and the system evolves according to law of physics (example by [Mike Bostock](http://bl.ocks.org/mbostock/4062045)).
 
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