Fix dataflow paper link [ci skip]

Signed-off-by: Paolo Di Tommaso <[email protected]>

README.md

@@ -1,7 +1,7 @@
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 *"Dataflow variables are spectacularly expressive in concurrent programming"*
-<br>[Henri E. Bal , Jennifer G. Steiner , Andrew S. Tanenbaum](http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.145.7873)
+<br>[Henri E. Bal , Jennifer G. Steiner , Andrew S. Tanenbaum](https://dl.acm.org/doi/abs/10.1145/72551.72552)
 
 
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